The Keeper's Vade-Mecum · Call of Cthulhu Reference

I · Overture

A Note Before the Game Begins

This is a working reference for the Keeper. Cosmology and lore lead; rules sit at the back as a quick-reference appendix. The voice is system-agnostic — most of the Mythos predates Chaosium and outlives any one edition. Where rules are quoted, they follow Call of Cthulhu 7th Edition (Chaosium, 2014).

Call of Cthulhu Keeper Rulebook cover (Chaosium, 7th Edition)

Call of Cthulhu Keeper Rulebook · Chaosium, 7th Edition

The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. — H.P. Lovecraft, “The Call of Cthulhu” (1928)

What Call of Cthulhu Is

Call of Cthulhu is an investigative horror roleplaying game in which ordinary people uncover supernatural evidence and pay for it with their wits, their bodies, and their reason. It runs on a percentile system: the Keeper sets a difficulty, the player rolls under their skill on a d100, and the world unfolds from the result. The numbers are simple. The damage they do is not.

The game was first published by Chaosium in 1981, designed by Sandy Petersen. Seventh edition (2014) is the current standard. Pulp Cthulhu (2016) layers two-fisted hero rules onto the base game. Several era-specific lines exist: Cthulhu by Gaslight (1890s), the default 1920s line, and the Modern Day setting most recently codified in the 7e core book.

How This Reference Is Organized

Run the Game — the Keeper's chair. Tone, pacing, sanity as drama, scenario structure, the question of when to spare investigators and when not to.

Cosmology — the cosmic order. Outer Gods, Great Old Ones, Elder Gods (with caveats), alien races, the Dreamlands, and an honest answer to the question every player eventually asks: what do they want with us?

Bestiary — servitor races, signature creatures, and the manifestable forms of the gods. Searchable, filterable. Stat blocks light, lore weighted.

Tomes & Spells — the forbidden books and a working sample of their contents.

Cults & Factions — the human side of cosmic horror. Who serves what, and why.

Eras — Gaslight (1890s), Classic (1920s), and Modern Day. Tone shifts, technology, how investigators communicate and travel, and what each era does to the horror.

Rules Reference — the 7e mechanical bones: skills, Sanity, combat, push rolls, bonus and penalty dice, Luck. Compressed for table use.

A Word on Canon

Mythos lore was built by many hands across a century. This reference flags provenance where it matters:

Lovecraft — H.P. Lovecraft's original stories (1917–1937).
Derleth — August Derleth's posthumous additions and reframings, especially the Elder Gods / good-vs-evil overlay and the elemental classifications. Often debated by purists.
Chaosium — material first appearing in the RPG line (avatars, scenario-canon cults like the Bloody Tongue, expanded statting).
C.A. Smith, Howard, Bloch, Campbell, Long, Bierce — named contributors to the Mythos canon, attributed where their creations appear.

You can run a single-tradition game (pure Lovecraft, for instance, leaving Derleth's Manichean overlay aside) or mix freely. The reference is built so you can find the seams and decide for yourself.

A Recommendation

If you have an evening and one investigator across the table, run The Haunting (the introductory scenario in the 7e Keeper Rulebook, also known as The Haunted House). Five hours, classic Boston haunting, every core mechanic gets exercised. There is a reason it has been the on-ramp since 1981.

II · The Keeper’s Chair

Run the Game

Call of Cthulhu lives or dies by atmosphere. The mechanics are servants. What the Keeper does is set tempo, hold the unknown, and let the players walk into it under their own steam. These are working principles, distilled from Chaosium's own Keeper guidance and decades of table practice.

The Keeper's chair — running Call of Cthulhu

Tone — Dread Over Shock

The greatest horror is the fear of the unknown. Don't describe every detail; leave room for players' minds to fill the gaps. Their imaginations will conjure worse than you can. When something terrible is about to happen, soften your voice. Players lean in. Then the moment lands.

Personal horror cuts deeper than monstrous horror. A familiar face wearing a wrong expression beats a tentacle in a basement. Sensory detail in the wrong key — a smell, a wet sound, an angle that refuses to make sense — is more disturbing than a full creature description.

An old technique: describe a glimpse of something to one player only, and when they ask for more detail, deny that you said it. Doubt is the open door horror walks through.

Pacing — You Are the Conductor

The Keeper's role is closer to an orchestra conductor than a referee. You set the tempo. That doesn't always mean fast. Players need quiet beats to compare notes, follow up clues, and rest before the next descent. A steady rhythm of investigation, revelation, and threat is what carries a session.

Resist the urge to volley Sanity loss in the first hour. A slow burn building to a crescendo is the form. The first session of a campaign should leave players unsettled, not broken.

Sanity at the Table

Describe the thing first. Make it frightening. Then, almost as an afterthought, ask for the Sanity roll. The mechanic should arrive after the dread, not in place of it.

When an investigator goes insane, work it out with the player. Ask them how their character breaks. Offer suggestions if they want them. The player keeps agency over their own collapse, which makes the collapse mean something.

Madness is dramatic, not punitive. A bout that becomes a phobia of mirrors is worth a hundred sessions of generic insanity. Build the breakdown into the character's ongoing arc.

The Three Phases of Madness (CoC 7e)

Bout of Madness — lasts 1d10 combat rounds (real-time) or 1d10 hours (off-screen summary). Player loses control of the investigator briefly. No further Sanity loss during the bout.
Underlying Insanity — any subsequent Sanity loss, even one point, triggers another bout. Lasts 1d10 hours (temporary) or until the scenario closes (indefinite).
Recovery — psychoanalysis, time, and (for indefinite cases) 1d6 months of institutional care. The character is changed, not erased.

Clues, Failure, and Moving the Story

If the investigators fail to discover a clue at one location, move the clue, or one like it, somewhere else. Or introduce a new clue that points them back to the place they missed. Investigation should not stall on a single failed Spot Hidden. The clue is the campaign's blood; keep it flowing.

Trust the players' intelligence. Resist the urge to volunteer information through Idea rolls when they haven't asked for one. Let them draw their own conclusions and surprise you with where they go.

Spot Hidden, Library Use, and Listen are the three skills players reach for most often. If a clue is gated behind any of these, have a backup path: a different skill, a different location, an NPC who finds them instead.

Death & Madness

Never kill an investigator when you can maim them. The fear of loss is more powerful than loss itself. Players who suspect their character could die at any moment play more cautiously, more cleverly, and more in-character than players who know the ax has already fallen.

If an investigator is about to walk into instant-death, warn them. Two or three escalating warnings — environmental wrongness, an NPC's fear, a direct “are you sure?” — before disaster strikes. Players who push past three warnings have chosen their fate, and they own it.

Isolate the party to raise tension. Cut them off from rescue, communication, or escape. Vulnerability is the texture of cosmic horror. They survive by their own action or not at all.

Scenario Shape — The Investigation Spine

Most Call of Cthulhu scenarios follow the same skeleton:

The Hook. An ordinary problem with a strange edge. A missing professor. A friend's letter that ends mid-sentence. An auction lot of antique books with one item that doesn't belong.
The Investigation. Three to five locations, each yielding clues that unlock the next. Aim for at least two paths between every pair of locations — horror collapses if the trail dead-ends.
The Revelation. The investigators understand what they're up against. This is when the largest Sanity loss often lands. The shape of the threat becomes clear, and clarity is its own horror.
The Climax. A choice with consequences: confront, banish, escape, or sacrifice. Combat is rare and lethal. Most climaxes are decided by a single ritual roll, a moral decision, or a desperate gambit.
The Aftermath. What was lost. What was learned. What follows the investigators into the next scenario. Some players want closure; some want the hook for next time. Offer both.

Prep Checklist

Before a session, confirm you have:

A one-line pitch for the scenario (the hook).
The threat's true nature, written in two sentences. (You don't need a paragraph; you need to know it cold.)
Three to five locations, each with at least one clue and one optional Sanity-loss moment.
Names — even thumbnail ones — for every NPC you'll voice. Improvising names mid-scene is the fastest way to break atmosphere.
One or two handouts. A telegram. A torn page. A photograph. A physical artifact at the table is worth a thousand words of description.
A list of Sanity-loss costs you've already decided. Don't improvise “0/1d6” under pressure; pre-set them.
An exit. Know how the scenario ends if the investigators run, fail, or die. Have at least one escape path baked in.

A Hard Rule

Don't make the players guess the truth of the Mythos. Make them witness it. Investigators piece together what an academic would write a book about — not solve a riddle. Information is the reward. Hiding the threat behind a maze of intentionally obscure clues is the surest way to kill a scenario.

III · The Cosmic Order

Cosmology & The Dark Lore

The Mythos is a cosmology of indifference. The universe is older, larger, and stranger than human minds were built to compass, and the things in it are not gods of any moral kind. Lovecraft called this view cosmicism. The horror flows from scale, not malice.

Cosmicism — The Underlying Frame

Cosmic indifferentism is the philosophical core. Humans are not the universe's protagonists, nor its enemies. We are bacteria on a slide that no one is examining. Sanity is a thin evolutionary accident, and exposure to the universe at its true scale shatters it not because the universe hates us, but because we were never meant to perceive it.

Some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the deadly light into the peace and safety of a new dark age. — H.P. Lovecraft, “The Call of Cthulhu”

This is the worldview that makes Call of Cthulhu work. Investigators don't lose because the gods are villains. They lose because the universe simply does not have a place for them, and walking far enough in its hidden corridors removes their place even from this world.

Derleth’s Overlay

August Derleth, after Lovecraft's death, recast the Mythos as a Manichean cosmic war: malevolent Outer Gods and Great Old Ones bound by long-dormant benevolent Elder Gods. He attached elemental classifications (Cthulhu = water, Cthugha = fire, Hastur = air) and family trees, and wrote stories of cosmic conflict resolved by rituals of opposition. Derleth material is enormously influential and not without merit, but it is not Lovecraft. Modern Chaosium play tends to lean back toward the original cosmicism. Use what serves your table; this reference flags Derlethian additions where they appear.

Outer Gods

Cosmic-tier entities. They embody principles or forces. Most are too vast or too alien to have any specific relationship with humanity. They cannot be meaningfully fought; in some cases they cannot even be coherently perceived.

Azathoth — The Blind Idiot God

Nuclear Chaos at the center of Ultimate Chaos.

Azathoth squats at the center of all things, surrounded by a court of mindless dancers and pipers. The universe may be his dream; if he wakes, or simply stops, reality ends. He has no will, no plan, no awareness. His name is invoked by sorcerers because the sound itself carries cosmic weight, but he is the least “personal” entity in the entire Mythos. Lovecraft

Featured in The Dream-Quest of Unknown Kadath (1943, written 1926–27) and referenced throughout the later stories. Cult presence on Earth is minimal and indirect — he is invoked, not served.

Yog-Sothoth — The Gate and the Key

Coterminous with all space and time.

Yog-Sothoth exists across all dimensions and moments simultaneously. He knows every past and future. To him space and time are not directions but a single object he is part of. Sorcerers bargain with him for knowledge, and occasionally for offspring — Wilbur Whateley of Dunwich was his half-human son. Lovecraft

Featured most directly in The Dunwich Horror (1929). The famous chant is his: “Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate. Past, present, future, all are one in Yog-Sothoth.” Cult presence: scattered occult sorcerers, hereditary bloodlines, and academic conjurers seeking forbidden knowledge.

Nyarlathotep — The Crawling Chaos

Messenger and soul of the Outer Gods. The active villain of the Mythos.

Nyarlathotep walks among humans. He enjoys it. Where Azathoth is mindless and Yog-Sothoth too vast to interact directly, Nyarlathotep is interested, malevolent, and hands-on. He has appeared as the Black Pharaoh, the Bloody Tongue, the Haunter of the Dark, the Black Man at witches' sabbaths, the Bloated Woman, the Crawling Mist, Ahtu, the Skinless One, and roughly a thousand other forms catalogued across Chaosium's expanded canon. Lovecraft Chaosium

He is the patron of more cults than any other Mythos entity: the Brotherhood of the Beast, the Cult of the Bloody Tongue, the Starry Wisdom Church, the Sand Bat Cult, and countless smaller cells. He delights in cultivating sorcerers and geniuses to ruin, in nudging civilizations toward catastrophe, in corrupting human institutions from within. If a scenario needs a recurring antagonist with a face, Nyarlathotep is the answer. From the prose-poem Nyarlathotep (1920) and most of Lovecraft's later cycle.

Shub-Niggurath — The Black Goat of the Woods With a Thousand Young

Fertility, propagation, and the perversion of both.

Shub-Niggurath is rarely seen directly. She is invoked, propitiated, and bred with. Her offspring include the Dark Young (tree-trunk-legged, mouthed, tentacled) and a vast and varied population of monstrous hybrid creatures. Her cults skew rural, witch-cult adjacent, and tied to fertility rites that go badly wrong. Lovecraft

Mentioned in The Whisperer in Darkness (1931), The Last Test, and others. Wilbur Whateley's grandfather invoked her in the same breath as Yog-Sothoth.

The Court of Azathoth & Lesser Outer Gods

Around Azathoth dance the Other Gods — mindless flute-pipers and capering forms, named individually only in Mythos-adjacent fiction. They are aspects of Ultimate Chaos rather than discrete entities. Daoloth, the Render of the Veils Campbell, is sometimes counted among the Outer Gods; observers of his unveiling glimpse the true geometry of reality and rarely return to use it. Ramsey Campbell created Daoloth as a post-Lovecraft addition; treat him as Outer Gods “reach” rather than core canon.

Great Old Ones

Powerful beings less vast than the Outer Gods. Most are confined to specific locations on or near Earth: imprisoned, dormant, or simply waiting. Unlike the Outer Gods, Great Old Ones can be statted (cautiously), bound, banished, or briefly defeated. None of them can be permanently destroyed.

Cthulhu

High Priest of the Great Old Ones; dreaming in sunken R’lyeh.

Cthulhu sleeps in the sunken city of R'lyeh in the South Pacific, awaiting the alignment of the stars. His dreams reach sensitive humans across the world, driving artists to fevered visions and cultists to acts of devotion. The famous couplet from the Necronomicon applies to him: That is not dead which can eternal lie, and with strange aeons even death may die. Lovecraft

His brief rising in 1925, recorded in The Call of Cthulhu (1928), ended when the steamer Alert rammed his head and the city sank again. He was reforming as the ship escaped. He cannot be killed. He can only be delayed.

Hastur the Unspeakable — He Who Must Not Be Named

Carcosa, Aldebaran, the Yellow Sign.

Hastur is the most genealogically tangled entity in the Mythos. Ambrose Bierce coined the name (in Haïta the Shepherd, 1893) for a benevolent shepherd-god. Robert W. Chambers borrowed it for The King in Yellow (1895), associating it with the lost city Carcosa, the Lake of Hali, and a play that drives readers mad. Lovecraft mentioned it in passing in The Whisperer in Darkness. Derleth and later Chaosium writers fused all three threads into a single Great Old One associated with decay, decadence, and the spread of madness through art. Bierce Chaosium Derleth

The Hastur = King in Yellow = Carcosa identification is contested canon. Use it freely; know it isn't pure Lovecraft. Speaking the name three times draws his attention; this is a Chaosium trope, not Lovecraft's.

Dagon & Hydra

Greater Deep Ones, ancient beyond reckoning.

Father Dagon and Mother Hydra are massively elder Deep Ones, worshipped as gods by their kin and by human cultists in coastal towns. Whether they are true gods or merely the eldest of their species is a question their cultists do not stop to ask. They serve Cthulhu and accept worship in exchange for gold, longevity, and breeding partnerships. Lovecraft

From Dagon (1919) and The Shadow Over Innsmouth (1936). The Esoteric Order of Dagon is their primary human cult.

Tsathoggua — The Sleeper of N’kai

Toad-bat-sloth thing of black Hyperborea.

Tsathoggua dwells in the lightless caverns of N'kai beneath the earth. Hyperborean sorcerers worshipped him; the Voormi of Mount Voormithadreth offered him sacrifice; the K'n-Yan of subterranean Oklahoma carry on a degenerate version of his cult. He is famously lazy, sometimes asleep for centuries, and accepts worship passively. His Formless Spawn do most of the dangerous work. C.A. Smith

Created by Clark Ashton Smith for his Hyperborean cycle (The Tale of Satampra Zeiros, 1931) and adopted into the Mythos with HPL's blessing. Featured in Lovecraft's The Mound (ghostwritten, 1940) and The Whisperer in Darkness.

Ithaqua — The Wind-Walker

Strider of arctic skies; the Wendigo by another name.

Ithaqua walks the polar wastes on long-striding legs, leaving no tracks. Those he carries off return weeks or years later, frozen and changed, sometimes as cultists, sometimes as something worse. Tied loosely to Algernon Blackwood's The Wendigo (1910) by association. Derleth

Created by August Derleth (in Ithaqua / The Thing That Walked on the Wind, 1933). Useful for arctic and northern-wilderness scenarios; reframes nature itself as actively hostile.

Cthugha

Living conflagration, dwelling near Fomalhaut.

Cthugha is a massive sentient flame summoned by repeating an invocation three times after midnight when Fomalhaut is above the horizon. Derleth's Manichean elemental scheme cast him as the fire-pole opposed to Hastur (air). He breeds Flame Vampires, lesser fire-elementals capable of independent action. Derleth

From The House on Curwen Street / The Dweller in Darkness (1944). Use freely if the elemental framework suits your campaign; ignore if it doesn't.

Yig — Father of Serpents

Half-human, half-snake; a folk-spirit god.

Yig is closer to a stern nature spirit than a typical Great Old One. He punishes those who harm snakes, and he rewards those who venerate them. His cult survives in pre-Columbian American traditions and among the Serpent People. He is rarely a primary antagonist; more often he is a transactional power, invoked by characters with very specific needs. Lovecraft

From The Curse of Yig (HPL/Bishop, 1929) and The Mound.

Atlach-Nacha

Spider-being weaving the bridge between worlds.

In a vast pit beneath Mount Voormithadreth, Atlach-Nacha spins a web that bridges the dream-world and the waking world. When the bridge is complete, the worlds merge and the world ends. She has been weaving for aeons. C.A. Smith

From Clark Ashton Smith's The Seven Geases (1934).

Y’golonac — The Defiler

Headless, palms-mouthed god of perversity and depravity.

Y'golonac is imprisoned behind a wall in a ruined city, but he reaches out to anyone who reads his name in the Revelations of Glaaki. He appears as a headless naked figure with mouths in the palms of his hands, offers his finder priesthood, and if accepted, possesses them and reshapes them into himself. Campbell

Created by Ramsey Campbell in Cold Print (1969). Excellent psychological-horror antagonist; the possession vector makes him dangerous out of all proportion to his combat ability.

Other Great Old Ones, in Brief

Bokrug, the Great Water Lizard of slumbering Sarnath. Ghatanothoa, sealed beneath Yaddith-Gho on lost Mu; his sight petrifies the body but leaves the mind alive within it. Rhan-Tegoth, frozen in the Arctic, requiring sacrifices or he withers; without him, no one can wake the others. Glaaki, dweller in a flooded English mere, attended by his half-living servants. Cyäegha, the Destroyer-Eye, sleeping beneath a German hill. Eihort, the brood-thing of the underworld. Many more — Chaosium's Malleus Monstrorum catalogues them at length.

Elder Gods

Derleth recast a small group of cosmic entities as benevolent opposition to the Outer Gods and Great Old Ones. The framing is non-Lovecraftian and is openly debated by purists. Most of the named “Elder Gods” are aloof, ancient, and not allies in any meaningful sense.

Nodens — Lord of the Great Abyss

Nodens is a real Romano-Celtic deity (a Nodens-cult survives at Lydney Park in Gloucestershire) whom Lovecraft borrowed and gave cosmic stature. He appears in The Strange High House in the Mist (1931) and rescues Randolph Carter from Nyarlathotep at the climax of The Dream-Quest of Unknown Kadath. He is the closest thing to a benevolent cosmic figure in Lovecraft's canon, but he is not a Derleth-style Elder God. He seems amused or annoyed by Nyarlathotep rather than engaged in cosmic war. He is served, in a fashion, by the Nightgaunts. Lovecraft

Bast, Hypnos & the Wider Elder Pantheon

Bast (the Egyptian feline goddess) and Hypnos (Greek god of sleep) appear lightly in Lovecraft's stories — Bast in The Cats of Ulthar (1920) by name, Hypnos in his eponymous tale (1923). Derleth elevated both to Elder God status. The canonical evidence for either as a cosmic combatant is thin; treat them as flavor or as deep-cut Derlethian additions. Vorvadoss, Ulthar the Unspoken, and other Derleth-only Elder Gods are deeper still in the Derleth wing of the cosmology.

Working Recommendation

If you run a pure Lovecraftian game, treat the Elder Gods as “not gods at all in the moral sense” — just very old, very removed entities who occasionally intersect with mortal affairs. The investigators are alone. If you run a Derlethian game, the Elder Gods can serve as the basis for banishment rituals, cosmic counter-magics, and rare hopeful endings. Both readings have decades of play behind them.

Alien Races — The Civilizations of the Mythos

Long-lived, technologically advanced, often alien-ethical species who have inhabited Earth or its near reaches. Several of them remember when humans were not the dominant species. A few are simply waiting to be again.

Elder Things (Old Ones of Antarctica)

Barrel-bodied, five-pointed, winged. They came to Earth around a billion years ago, built vast cities under what is now Antarctica, and engineered the shoggoths as living tools. Their civilization fell when the shoggoths revolted and the climate shifted. A handful sleep in deep ice. Lovecraft writes of them with unexpected sympathy: “Scientists to the last — what had they done that we would not have done in their place? … they were men!” Lovecraft

From At the Mountains of Madness (1936). The most morally complex alien species in the Mythos.

The Great Race of Yith

Currently inhabiting cone-shaped bodies in pre-human Australia, around 150 million years ago. The Great Race is a mind-collective: when their host species faces extinction, they project their consciousness across time into a new species, displacing the natives' minds into the bodies they leave behind. They are scholars, archivists, and the keepers of the largest historical library in cosmic history. They will jump again before Earth dies. Lovecraft

From The Shadow Out of Time (1936). Not malevolent — merely curious, methodical, and willing to overwrite a human mind for a few years of research.

Mi-Go — Fungi from Yuggoth

Crustacean-fungal hybrids with membranous wings that work in vacuum. They mine rare elements from the hills of Earth, especially Vermont, and maintain outposts on Yuggoth (Pluto). Their signature technology is the brain cylinder — a metal canister that houses a living human brain with sensory attachments, allowing transport across interstellar distances. They worship Nyarlathotep and Shub-Niggurath, and trade transactionally with humans they cannot quietly silence. Lovecraft

From The Whisperer in Darkness (1931). The most adversarially-oriented Mythos race in active interaction with humans.

Deep Ones

Aquatic humanoids, immortal unless killed. They interbreed with humans, producing offspring who appear human in youth and gradually transform into full Deep Ones, eventually descending to undersea cities. Y'ha-nthlei off Devil Reef is the famous one. They worship Father Dagon, Mother Hydra, and Cthulhu. The Esoteric Order of Dagon brokers the Innsmouth bargain. Lovecraft

From The Shadow Over Innsmouth (1936).

Ghouls

Canine-hooved, rubbery-skinned cemetery dwellers. They eat the dead, bargain with the living, and occasionally adopt human children to raise as their own. Humans can become ghouls over time — the painter Richard Pickman is canon-famous for it. Crucially, ghoul tunnels connect waking-world graveyards to the Dreamlands; many ghouls move between both. Lovecraft

From Pickman's Model (1927) and extensively in The Dream-Quest of Unknown Kadath. They are not always hostile; a starving or threatened ghoul, however, is lethal.

Serpent People

A pre-human reptilian civilization centered on Valusia. Originally Robert E. Howard's, in his King Kull stories, and absorbed into the Mythos. They are masters of sorcery and capable of illusion that lets them pass for human. The civilization fell long ago; survivors and hybrids persist in deep caverns and scattered cult-cells, plotting humanity's overthrow. They worship Yig and sometimes Tsathoggua. Howard

K’n-Yan

A subterranean blue-skinned humanoid civilization beneath Oklahoma. Telepathic, immortal, decadent, and bored to a degree that has made them cruel. They worship Tulu (Cthulhu), Yig, and Tsathoggua. Below them lies red-lit Yoth (extinct serpent-people city) and lightless N'kai (Tsathoggua's lair). Lovecraft

From The Mound (HPL/Bishop, 1940).

Voormi & Other Lost Hominids

The Voormi were three-lobed-eyed shaggy proto-hominids of Hyperborea, worshippers of Tsathoggua. C.A. Smith Mostly extinct; degenerate remnants may persist in remote mountains. Other lost hominids appear in Mythos fiction (Ib's batrachian inhabitants, the night-gaunts of the Dreamlands) at the boundary between alien race and creature.

What Do They Want With Us?

This is the question every player asks the first time the Sanity die comes up. The honest answer is one of the most-misunderstood parts of the Mythos.

Almost nothing. Outer Gods and most Great Old Ones are too vast, too alien, or too indifferent to want anything from humanity specifically. We are background noise. There is no harvest, no plan that requires us, no contract we are unknowingly fulfilling. Cosmic indifferentism is the whole frame.

Why, then, do humans serve them?

Madness. Exposure to cosmic truth breaks people, and broken people sometimes worship.

Ambition. Sorcerers like Joseph Curwen, Wilbur Whateley's grandfather, and Keziah Mason want power, longevity, escape from death. They can purchase these things at extraordinary cost.

Hereditary corruption. The Innsmouth bloodlines, the Whateleys, the Marsh family. Once the contamination is genetic, the descendants did not choose anything.

Inherited tradition. Isolated populations — the Greenland cultists in The Call of Cthulhu, the Louisiana swamp-cells, scattered island and mountain communities — preserve worship across millennia as folk practice. The original reasons are lost.

The exceptions — entities that do interact

Nyarlathotep is the great exception. He walks among humans in human guise, founds cults, manipulates events, and seems to enjoy human degradation and chaos. He is the active, scheming antagonist of the Mythos. If a campaign needs a villain who plans, he plans.

Yog-Sothoth grants knowledge, opens gates, and sires hybrid children to summoners who pay the price.

Shub-Niggurath produces hybrid offspring and accepts fertility-cult worship.

Hastur, in expanded Chaosium canon, infects through art, theatre, and the play of the King in Yellow.

Yig punishes harm to snakes and rewards veneration — transactional, almost folkloric.

When the stars are right

This is the closest thing to an Outer God plan. From The Call of Cthulhu:

When the stars came right again… the secret cult would always be waiting to liberate them. Then, the liberated Old Ones would teach them new ways to shout and kill and revel and enjoy themselves, and all the earth would flame with a holocaust of ecstasy and freedom. — H.P. Lovecraft

The shape of this is not a harvest. It is a transformation, or a destruction, of the world humans currently inhabit. The cult does not necessarily survive what it ushers in. They want to be present at the end, not survive into the next age.

The Soul Question

The Mythos is fundamentally materialist. Lovecraft was an atheist and a mechanist. There is no Christian-style soul to harvest, no afterlife marketplace, no divine accounting. Mind, in the Mythos, is a physical and informational pattern in a brain. So:

Mi-Go transport brains in literal metal cylinders. The brain is the person.
Yithians swap minds — informational patterns — between bodies across time. Mechanistic, not spiritual.
Hounds of Tindalos consume their prey through angles in time. They eat something — life-force, mind, flesh — but as predators, not soul-collectors.
Deep Ones transform humans physically, not spiritually. The Innsmouth taint is genetic.
Asenath / Ephraim Waite in The Thing on the Doorstep performs body-swap by sorcery. This is presented as a learned magical technique, not divine soul-theft.

When Chaosium scenarios talk about “souls being taken,” that is dramatic shorthand or post-Lovecraft drift. Pure Lovecraft has no such mechanic. The horror in his stories is that there is no soul to lose — only a mind, and the mind can be erased or rewritten or eaten without a celestial witness. That is part of what makes the cosmicism land.

Run with the framing your table needs. If your players want a soul stake, the Mythos can accommodate one (and Derleth-era material happily does). If they want the bleaker materialism, that is closer to the source.

The Dreamlands

A parallel realm reached by skilled dreamers descending the seventy steps of light slumber, then the seven hundred steps of deeper slumber, past the priests Nasht and Kaman-Thah. The Dreamlands are tonally distinct from the cosmic-horror Mythos — closer to Lord Dunsany's fantasy than Lovecraft's later work — but cosmic horror leaks in.

Notable places: Celephaïs in Ooth-Nargai, the eternal city. Ulthar, where by ancient law no man may kill a cat. Dylath-Leen, the obsidian port where the black galleys of the moon-beasts dock. Sarkomand, the ruined city. Inquanok, of black quarries. Leng, the cold waste with its corpse-eating high-priest in yellow silk. Kadath in the Cold Waste, with its onyx castle of the Great Ones — Earth's milder gods, watched over by Nyarlathotep. Below all of it, the Underworld, with the Vale of Pnath and the realms of the ghouls.

Foundational stories: The Dream-Quest of Unknown Kadath (1943), Celephaïs (1922), The Cats of Ulthar (1920), The Other Gods (1933), The White Ship (1919). Randolph Carter is the recurring protagonist across the dream cycle.

Chaosium publishes a Dreamlands sourcebook (multiple editions); use it as a softer fantasy adjunct, a bait-and-switch into deeper horror, or a place where investigators trapped in the waking world can pursue parallel quests through dream.

IV · Of Things That Move and Things That Wait

Bestiary

Servitor races, signature creatures, and the manifestable forms of the gods. Stat blocks reflect Call of Cthulhu 7th Edition averages, drawn from the Keeper Rulebook and Malleus Monstrorum. Sanity loss values are listed prominently because they are usually the most consequential thing on the page.

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V · The Forbidden Library

Tomes & Spells

A working sample of the Mythos library and the spells investigators are most likely to encounter. Reading any of these books costs Sanity, takes time, and grants Cthulhu Mythos points (which raise the ceiling of cosmic insight while lowering the ceiling of maximum Sanity by the same amount). Spells are entries; tomes are the libraries from which entries are drawn.

The Necronomicon

The most famous Mythos tome and the foundational grimoire. Written in Damascus by Abdul Alhazred (the “Mad Arab”) in approximately 730 CE, originally titled Al Azif. Translated into Greek as Necronomicon by Theodorus Philetas in 950 CE, into Latin by Olaus Wormius in 1228, and into English (in incomplete form) by Dr. John Dee in the late 1500s. Lovecraft

It is the source of the famous couplet:

That is not dead which can eternal lie,
And with strange aeons even death may die.

Surviving copies are exceptionally rare. The British Museum holds one. Miskatonic University holds the John Dee English fragment. The Bibliothèque Nationale, the Widener Library at Harvard, and the University of Buenos Aires are also said to hold copies. Reading the complete Latin or Greek text is months of work and represents catastrophic Sanity exposure.

Other Major Tomes

Pnakotic Manuscripts

Pre-human in origin, partly Yithian. Covers the deepest cosmic history available to mortal readers. Featured in Polaris (1920) and The Other Gods. Fragments survive in scattered collections; the Pnakotic library itself exists, but in the Dreamlands, in lost Lomar. Lovecraft

Book of Eibon (Liber Ivonis)

Hyperborean wizard Eibon's grimoire, focused on Tsathoggua and the sorceries of lost Hyperborea. Original Hyperborean lost; surviving copies are translations of translations. C.A. Smith

Unaussprechlichen Kulten (Nameless Cults)

Friedrich Wilhelm von Junzt's 1839 survey of global cult activity, suppressed almost immediately on publication. Von Junzt was strangled in his locked study a year later by something that left no other tracks. Howard

De Vermis Mysteriis (Mysteries of the Worm)

Ludvig Prinn's 16th-century compendium of summoning, demonology, and worm-cult lore, written shortly before he was burned at the stake by the Inquisition. Bloch

Cultes des Goules

The Comte d'Erlette's 17th-century French treatise on ghoul cults and necromancy. (D'Erlette was Derleth's family-name self-insertion — mostly affectionate.) Derleth

R’lyeh Text

Pre-human, the primary Cthulhu-cult source document. Written in characters and a language no human mind should attempt. Translation is nigh-impossible; possession alone marks the bearer.

Celaeno Fragments

Notes brought back by Laban Shrewsbury from the great library on Celaeno, a moon in the Pleiades, copied from far older originals. Modern; mid-twentieth century in canon. Derleth

The King in Yellow

A play in two acts, published in multiple languages and quietly censored everywhere. The first act is unremarkable. The second act drives readers mad. Themes: Carcosa, the Lake of Hali, the Yellow Sign, the Pallid Mask, the Phantom of Truth. Chambers — from Robert W. Chambers's 1895 collection of the same name; included with Bierce-tag because Hastur and Carcosa originate with Bierce.

Revelations of Glaaki

An accreted, partly hand-copied British grimoire of the cult of Glaaki in the Severn Valley. Reading it can summon Y'golonac. Campbell

A Working Sample of Spells

CoC magic costs Magic Points (POW), Sanity, and sometimes hit points or material components. Most spells take rounds, minutes, or hours to cast. Spells are not balanced game mechanics; they are dangerous, expensive, and shape scenarios more than combat. The list below is representative, not exhaustive.

Spell	Cost	Effect
Contact Deity / Servitor (numerous variants)	1+ Magic Points, 1d10 SAN	Briefly contacts a named entity. Outcome depends entirely on the entity. Cthulhu, Nyarlathotep, Mi-Go, Deep Ones, Ghouls, etc., each have their own version.
Summon / Bind <Servitor>	Variable MP, 1d6+ SAN	Summons a Mythos creature (Byakhee, Dimensional Shambler, Star Vampire, etc.). Binding the summoned creature to obey the caster is a separate, opposed test.
Elder Sign	1 MP, 1 SAN	Inscribes the Elder Sign — a barrier against many Mythos entities. The exact icon varies by tradition; Chaosium's is a five-armed branch-like glyph.
Voorish Sign	1 MP per use	A gesture that briefly reveals invisible Mythos entities and aids in detection.
Dread Curse of Azathoth	Variable MP, 1d10 SAN	Curse a target with a slow, unraveling doom. One of the iconic offensive spells of Mythos sorcery.
Mind Transfer	~16 MP, 1d20 SAN	Swap minds with another individual. The signature working of Asenath / Ephraim Waite, and (mechanically simulated) of the Yithians.
Powder of Ibn-Ghazi	Material; ritual prep	Reveals invisible creatures temporarily. Famous from The Dunwich Horror; thrown into the air around the suspected presence.
Resurrection (Essential Saltes)	Materials, weeks, 1d10 SAN per ritual	Joseph Curwen's working: distill a person's bodily salts from their grave, raise them as fully-functioning intelligent corpse. Famously demonstrated in The Case of Charles Dexter Ward.
Call / Dismiss <Outer God or Great Old One>	Catastrophic	Genuinely summons the entity. Almost always a scenario-ending event. Dismiss is the corresponding banishment. Both are typically the climactic ritual of an entire campaign arc.

Reading the Books — The Mechanic, In Brief

In Call of Cthulhu 7e, every Mythos tome has three values: an initial reading time, a Sanity loss for skimming, and a Sanity loss for full study. Reading a tome grants:

Cthulhu Mythos points — raise the investigator's Cthulhu Mythos skill.
Reduced Sanity ceiling — maximum Sanity equals 99 minus the investigator's Cthulhu Mythos skill. Mythos knowledge displaces sanity permanently.
Spell access — specific spells the tome contains. Each spell is its own time investment to learn.

This is the central irony of the game's economy. Investigators need Mythos knowledge to fight back, and gaining it makes them less able to remain themselves. There is no path through the Mythos that does not cost mind for understanding.

VI · The Human Cost

Cults & Factions

The human side of cosmic horror. Mythos cults persist in concentric rings: outer initiates who suspect nothing, inner devotees who know enough to be useful, and the small core that has seen what it serves and continues anyway. Most cult members are people. That is what makes them dangerous.

The Cthulhu Cult

Global, ancient, decentralized. Cells operate independently across millennia: Esquimaux idol-cultists in Greenland, swamp-dwelling worshippers in the Louisiana bayous, South Pacific islanders, Scottish coastal communities, scattered cells in every port city of importance. No central hierarchy, no unified theology — just the same dream, the same waiting, the same patient certainty that the stars will come right and the sleeper will wake. Lovecraft

Featured in The Call of Cthulhu (1928). The chant: Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn — “In his house at R'lyeh, dead Cthulhu waits dreaming.”

The Esoteric Order of Dagon

The civic-religious institution of Innsmouth, Massachusetts. Founded by Captain Obed Marsh in the early 1800s after his trade contacts with Polynesian Deep One cultists led him to broker a similar arrangement at home. Members marry Deep Ones, receive gold and longevity in exchange for offspring, and, in old age, descend to the undersea city of Y'ha-nthlei to live forever as Deep Ones themselves. Federal raids in 1928 disrupted the Order; the bloodlines persist. Lovecraft

From The Shadow Over Innsmouth (1936). The single most playable cult in all of Call of Cthulhu — a closed New England town with a hereditary supernatural inheritance.

The Brotherhood of the Beast

Egypt-rooted cult of Nyarlathotep in his Black Pharaoh / Nephren-Ka avatar. Ancient: traces to dynastic Egypt and the heretic pharaoh of the same name. Modern cells operate as a multinational secret society pursuing the resurrection of the Black Pharaoh and the reordering of the world to match prophecy. Heavily expanded in Chaosium's Day of the Beast and Masks of Nyarlathotep. Chaosium

The Starry Wisdom Church

A Providence, Rhode Island cult of the Haunter of the Dark (a Nyarlathotep avatar). The cult's headquarters was the Free Will Church on Federal Hill; their central artifact, the Shining Trapezohedron, summons the Haunter when contemplated in darkness. The cult was officially suppressed in the 1870s after public outrage but persists in scattered cells. Lovecraft

From The Haunter of the Dark (1936).

The Cult of the Bloody Tongue

Kenyan Mountain of the Black Wind cult, worshippers of Nyarlathotep's Bloody Tongue avatar. Largely a Chaosium expansion of Lovecraftian hints, made central by the Masks of Nyarlathotep campaign (Chaosium, 1984; one of the most acclaimed RPG campaigns of all time). Chaosium

The Sand Bat Cult

Australian Outback cult, an avatar of Nyarlathotep. Almost entirely Chaosium-canonical, developed for the Australian leg of Masks of Nyarlathotep. Chaosium

The Cult of Yig

Pre-Columbian American snake-cults, mountain-dwelling traditions in Central and South America, scattered survivor cells in the modern American Southwest. Yig's worshippers are not always sinister; some are simply farmers and herders honoring an ancient covenant not to harm snakes. The malevolent cells exist, but Yig's cult is more folkloric than apocalyptic. Lovecraft

Cthulhu Mythos Characters · TVTropes

Witch-Cults of New England

Salem, Arkham, and the wider New England witch-cult tradition. Keziah Mason of Arkham is the canonical figure (The Dreams in the Witch House, 1933). Witch-cults blend folk magic, Wilbur-Whateley-style sorcery, and contact with the Black Man (a Nyarlathotep avatar). Multi-generational; inheritance often runs through bloodlines. Lovecraft

The Order of the Yellow Sign

Decadent aesthete-cults of Hastur and the King in Yellow. Theatre groups quietly performing the play. Salons of nihilist intellectuals. Fin-de-siècle artistic circles touched by the wrong influences. The Yellow Sign appears in their margins, on their letterheads, on the covers of their books. Chaosium Chambers

The Chosen of Glaaki

Severn Valley cultists who have been “blessed” by Glaaki's spines, becoming his half-living servants. Long-lived, intelligent, increasingly inhuman. The Revelations of Glaaki is their gospel. Campbell

Modern-Day Cults

The Mythos has not gone away in the present. Modern Day cults take the shape of fringe-religious movements, online occult subcultures, doomsday-prepper compounds with stranger contents than expected, and corporations whose unusual research divisions look like cults to anyone close enough to see. Chaosium's modern-era supplements (especially The Two-Headed Serpent and Cthulhu Now derivatives) flesh these out. Chaosium

VII · Three Centuries of Horror

Eras of Play

Call of Cthulhu has three official eras, each with its own published line and tonal personality. The Mythos itself is unchanged across them; what changes is the world investigators must function in — the technology they have to help, the speed of communication, and the quality of the lights they can turn on against the dark.

Three eras of Call of Cthulhu — Gaslight, Classic, Modern Day

Cthulhu by Gaslight

1880s – 1900s · Late Victorian / Edwardian

Gaslight investigators move through fog-thickened London, the salons of fin-de-siècle Paris, the spiritualist parlors of New York, and the vast colonial reach of the European empires. Telegraph and steam railway are the fastest information and travel; gaslight is the fastest reliable artificial light. Telephones exist by the late 1880s but are rare. The Necronomicon still lives in private collections that have not yet been catalogued. The Theosophical Society was founded in 1875; the Hermetic Order of the Golden Dawn in 1888. Spiritualism is fashionable. Egyptology is the popular science of the moment.

Tone. Gothic-edged horror with class consciousness. The world is large but well-mapped; the empire's blind spots are where the Mythos festers. Investigators are often gentlemen-scholars, alienists, ex-military officers, suffragettes, journalists, and members of newly-formed occult societies.

Horror notes. The slow communication and uneven technology make isolation easy. A telegram takes hours; a transatlantic letter takes a week. The investigators stranded at a country house with the snow setting in is a Gaslight set-piece.

Hooks. The disappearance of an Egyptologist after returning with a private collection. A spiritualist medium whose recent séances have produced something that is not what is being requested. A new edition of a long-suppressed book appearing in a bookshop in Bloomsbury. Strange murders in Whitechapel that bear marks no pathologist will officially record.

The Classic Era (Lovecraft Country)

1920s · The Default Setting

The default Call of Cthulhu setting and Lovecraft's own period. Prohibition-era America, post-war Europe, the colonial world cracking at its seams. Telephones, automobiles, transatlantic liners, radio, and early commercial aviation. Cinema is silent through 1927, then talking. Hieroglyphic writing has been deciphered; Howard Carter opened Tutankhamun's tomb in 1922. The lost-civilization fascination is at its peak. The League of Nations exists; the Treaty of Versailles is recent; the world is being redrawn.

Tone. The classic Mythos voice. Universities, antiquarian collections, expedition narratives, jazz clubs, country estates with bad histories. Investigators are professors, journalists, dilettantes, doctors, criminals, and the occasional working person who knows too much.

Horror notes. The 1920s are a deceptively modern setting — fast, wealthy, cosmopolitan, well-connected by Gaslight standards. The horror works because the modernity is brittle. Outside the city limits, nothing has changed in centuries. Innsmouth is fifty miles from Boston. Dunwich is a hundred. The trains that carry investigators between New York and Buenos Aires take weeks; the trail goes cold easily.

Hooks. A friend's letter posted from the Australian outback that ends mid-sentence. A Miskatonic expedition that did not return from Antarctica. An auction lot of antique books with one volume that should not exist. A small Massachusetts town with too few children and too many fish-eyed elders.

Modern Day

2000s – Present · The Connected World

Modern Day investigators have the internet, smartphones, GPS, instant translation, satellite imagery, and the entire searchable corpus of human knowledge in their pockets. They also have all of these things' shadows: surveillance, algorithmic visibility, deepfakes, cancellation, and the impossibility of truly disappearing. The Mythos persists in places the global panopticon does not yet reach — or hides in plain sight in places the panopticon refuses to look.

Tone. Paranoia in a connected world. Information moves at light speed; horror moves faster than information's ability to verify it. Fringe research forums, cult subreddits, deep-web archives, leaked corporate research, and inexplicable satellite imagery are the new tomes. Investigators are journalists, academics, hackers, federal agents, private investigators, conspiracy theorists, and the occasional ordinary person who took the wrong photograph.

Horror notes. The connected world flattens many traditional Mythos isolation tropes. Compensate by reframing the threat: cell signals fail at the worst time, GPS lies near certain places, the cloud loses your evidence overnight, the witnesses you talked to yesterday have no memory of the conversation today. The Mythos in modern setting is a virus in the information layer as well as the physical one.

Hooks. A subreddit dedicated to a missing person fills with members who don't seem to be human. A forensic archaeology team's drone footage of a remote dig is corrupted in identical ways across every backup. A pharmaceutical company's safety division hires the investigators to look into a research site that has stopped reporting. A friend's last text message arrives three days after they disappeared.

Tone Across All Eras

Whatever the era, the texture of cosmic horror is the same: small lives intersecting cosmic forces, ordinary tools failing against extraordinary problems, and the slow realization that what is wrong with the world is not local. The era determines what the lights look like, what the maps say, and how fast the news travels. The dark beneath all of them is the same dark.

VIII · The Mechanical Bones

Rules Reference

A condensed working reference for Call of Cthulhu 7th Edition. This is enough to run a session at the table; it is not a substitute for the Keeper Rulebook. Rules quoted here follow Chaosium 2014, with footnoted corrections from later printings where they exist.

Characteristics

Eight characteristics, each rated 1–99 (typically 15–90 for humans). Investigator creation rolls them as percentile values directly.

Stat	Roll	Range	What It Measures
STR	3d6 × 5	15–90	Physical strength.
CON	3d6 × 5	15–90	Physical resilience.
SIZ	(2d6+6) × 5	40–90	Mass & height.
DEX	3d6 × 5	15–90	Reflexes & coordination.
APP	3d6 × 5	15–90	Appearance & charisma.
INT	(2d6+6) × 5	40–90	Reasoning, memory, problem-solving.
POW	3d6 × 5	15–90	Willpower, magical aptitude. Equals starting Sanity.
EDU	(2d6+6) × 5	40–90	Schooling & learned knowledge.
Luck	3d6 × 5	15–90	Fate's loose change. Spendable; recoverable between sessions.

Derived stats: Hit Points = (CON + SIZ) / 10, rounded down. Sanity = POW (max 99 minus Cthulhu Mythos skill). Magic Points = POW / 5. Move (MOV): 9 if STR & DEX > SIZ, 7 if both < SIZ, otherwise 8.

Damage Bonus & Build (STR + SIZ)

STR + SIZ	Damage Bonus	Build
2–64	−2	−2
65–84	−1	−1
85–124	None	0
125–164	+1d4	+1
165–204	+1d6	+2
205–284	+2d6	+3
285–364	+3d6	+4

Skills (Selected, with Base %)

Investigators improve skills via point allocation at character creation (drawn from EDU and personal/occupation points) and via successful use during play. The full 7e skill list runs to roughly eighty entries; the most-used are below.

Accounting 05

Anthropology 01

Archaeology 01

Charm 15

Climb 20

Computer Use 05

Credit Rating 00

Cthulhu Mythos 00

Disguise 05

Dodge DEX/2

Drive Auto 20

Electrical Repair 10

Fast Talk 05

Fighting (Brawl) 25

Firearms (Handgun) 20

Firearms (Rifle/SG) 25

First Aid 30

History 05

Intimidate 15

Jump 20

Language (Other) 01

Language (Own) EDU

Law 05

Library Use 20

Listen 20

Locksmith 01

Mechanical Repair 10

Medicine 01

Natural World 10

Navigate 10

Occult 05

Persuade 10

Psychoanalysis 01

Psychology 10

Ride 05

Sleight of Hand 10

Spot Hidden 25

Stealth 20

Survival 10

Swim 20

Throw 20

Track 10

Skill Rolls & Difficulty Levels

Roll d100; equal-or-less-than skill is a success. The Keeper sets the difficulty level based on circumstance:

Difficulty	Target	When To Use
Regular	Skill value	Normal use under typical conditions.
Hard	½ skill value	The challenge would defeat someone with average ability in the skill.
Extreme	⅕ skill value	The challenge would defeat all but the most skilled.
Critical	01	Always succeeds. Best possible outcome.
Fumble	100 (or 96–100 if skill < 50)	Catastrophic failure. Worst possible outcome.

Push Rolls

After a failed skill or characteristic roll, the player may justify pushing the roll — declaring a more determined, more drastic, or more time-consuming attempt. The Keeper either accepts the justification (allowing one re-roll) or rules the push impossible. A failed push has dire consequences. The investigator who pushes a Library Use to find a buried reference, fails, and has the librarian return to find them at the wrong shelf has compounded the original failure into a discovered intrusion. This is the design.

You cannot push: combat rolls, damage rolls, Sanity rolls, Sanity-loss rolls, or Luck rolls. Some characteristic rolls (CON to resist disease, POW to resist a spell) cannot be pushed by Keeper ruling. Combat is final at the moment of resolution — a missed punch is a missed punch; the next punch is a new opportunity, not a re-roll.

Bonus & Penalty Dice

For circumstances that aid or hinder a roll, the Keeper grants a bonus or penalty die. Roll an additional tens d10 (alongside the normal d100), then choose one to use:

Bonus die: use the lower of the two tens results. Favor the player.
Penalty die: use the higher of the two tens results. Punish the player.

One bonus and one penalty die cancel. Up to two of each may stack. Bonus and penalty dice never apply to Sanity rolls or Sanity-loss rolls.

Luck

After a failed skill or characteristic roll (not a Sanity, damage, or Luck roll), the player may spend Luck points one-for-one to reduce the roll's result to a value at or below their skill. A roll that fumbled cannot be saved by Luck; a critical cannot be purchased.

If you push the roll, you cannot also spend Luck on it. Pick one. Either gamble on a re-roll, or pay to retroactively succeed. Luck is recovered between sessions: roll 1d100; if the result exceeds your current Luck, gain 1d10 Luck (capped at 99).

Combat

Combat rounds are loose units of time, around twelve seconds, in which every combatant gets one action. Order is highest DEX first; ties go to higher combat skill. An action is one of: attack, maneuver, flee, cast spell, or other timed action.

Melee resolution. Attacker and defender make opposed rolls. The defender chooses Dodge (base DEX/2) or Fight Back (their own combat skill). Higher level of success wins. Ties when defender Dodges go to the defender; ties when defender Fights Back go to the attacker. Both fail: no damage on either side.

Extreme success on a melee attack on the attacker's own turn: deals maximum damage. With a penetrating weapon, this becomes an Impale — max damage plus an additional damage roll.

Maneuvers (grapples, disarms, knockdowns) compare Build values: a defender of smaller Build by 1 takes a penalty die, by 2 two penalty dice, by 3+ cannot resist at all.

You cannot push a combat roll. You missed; that swing is over; next round is a new attack.

Firearms (Quick Reference)

Range bands: Regular at base range, Hard at 2× base, Extreme at 4× base. Beyond 4× you cannot hit.
Point-blank (within DEX/5 in feet): +1 bonus die.
Aiming (declare, hold for next round, take no other actions): +1 bonus die.
Cover/concealment (target half-obscured or more): +1 penalty die.
Fast-moving target (MOV 8+): penalty die.
Build −2 or smaller target: penalty die. Build 4 or larger: bonus die.
Multiple shots in a round: penalty die on each shot after the first.
Firing into melee: penalty die. A fumble hits an ally (the lowest Luck takes it, if multiple).
Targets cannot Dodge or Fight Back firearm attacks the way they can melee. They can take cover, drop prone, or fire back as their own action.
Reloading: one round to load 2 shells in a handgun/rifle/shotgun. One round to swap a clip. Two rounds for a machine-gun belt.
Full auto: a volley of (skill / 10, round down, minimum 3) bullets. Each subsequent volley adds another penalty die.

Sanity

Sanity (SAN) starts equal to POW. Maximum Sanity is 99 minus the investigator's Cthulhu Mythos skill. Sanity loss rolls are written as X / Y — X on a successful Sanity roll, Y on a failed roll.

Reading the format: “0/1d6” means no loss on success, 1d6 loss on failure. “1/1d6+1” means 1 loss on success, 1d6+1 on failure. A fumbled Sanity roll incurs maximum loss. Bonus and penalty dice never apply.

Sample Sanity Costs

Trigger	Cost
Mangled animal carcass	0/1d2
Corpse or body part	0/1d3
Stream of blood	0/1d4
Mangled human corpse	1/1d4+1
Awake in a coffin / friend's violent death / see a ghoul	0/1d6
Meet someone known to be dead	1/1d6+1
Severe torture	0/1d10
Corpse rises	1/1d10
Severed head falls from sky	2/2d10+1
See Great Cthulhu	1d10/1d100

Insanity Thresholds

Temporary insanity: losing 5+ Sanity from a single source triggers an INT roll. Failing the INT roll means the investigator represses the experience; no insanity follows. Passing the INT roll means the investigator grasps what they have seen, and goes temporarily insane for 1d10 hours. (Passing INT is the bad outcome — ironic but canonical.)
Indefinite insanity: losing one-fifth or more of current Sanity in a single game day. Lasts until cured or recovered, often months.
Permanent insanity: Sanity reaches 0. The investigator is removed from play.

Bouts of Madness — Real Time (1d10 Combat Rounds)

Roll 1d10:

Roll	Effect
1	Amnesia — loses memory of the immediate event.
2	Psychosomatic disability — blindness, paralysis, mutism (random).
3	Violence — lashes out at nearest target.
4	Paranoia — everyone is an enemy.
5	Significant person — reverts to a powerful relationship.
6	Faints.
7	Flees in panic.
8	Hysterics — uncontrolled laughing, weeping, screaming.
9	Phobia — develops a new phobia (Keeper picks).
10	Mania — develops a new mania (Keeper picks).

Recovering Sanity

Psychoanalysis: once per game month. Successful skill roll grants 1d3 Sanity. Fumble loses 1d6 Sanity and ends the therapy.
Indefinite insanity: requires 1d6 months of dedicated treatment, often institutional.
Backstory anchors: spending time on a meaningful relationship, hobby, or training (defined in the investigator's backstory) can restore 2d6 Sanity at major milestones.
Maximum Sanity: 99 minus Cthulhu Mythos skill. Mythos knowledge displaces Sanity permanently.

A Compressed Combat Round — Worked Example

Investigator Hayes (Fighting Brawl 50, Dodge 35, DEX 60, Build +1, DB +1d4) faces a single Deep One (Fighting Brawl 50, Dodge 35, DEX 65, Build +1, DB +1d4).

Initiative: Deep One acts first (DEX 65 vs 60).
Deep One attacks claw at Hayes; Hayes elects to Dodge. Both roll d100. Deep One: 38 (Regular success). Hayes: 27 (Regular success). Tie when Hayes Dodges — Hayes wins; no damage.
Hayes's turn. Strikes back with brawl. Deep One Dodges. Hayes: 22 (Regular). Deep One: 41 (failure). Hayes hits, deals 1d3+1d4 = 5 damage. Deep One has 13 HP — now 8.
Round ends. New round, both act again. The Keeper notes the Deep One's webbed companions are sliding out of the harbor a hundred yards back.

For full rules — chases, magic-point combat, sanity recovery edge cases, the bestiary chapter — consult the Call of Cthulhu Keeper Rulebook, 7th Edition (Chaosium, 2014) and the Investigator Handbook (2014).