Call of Cthulhu RPG: The Cthulhu Mythos

Cosmic Entities and Forbidden Knowledge

The Universe According to Lovecraft

The Cthulhu Mythos is not just a collection of monsters and magic spells - it's a complete cosmological framework that redefines humanity's place in the universe. Imagine discovering that everything you thought you knew about reality, history, and the nature of existence was not just wrong, but laughably, pathetically incomplete. The Mythos presents a universe where humanity is neither the pinnacle of evolution nor the center of cosmic attention, but rather an accidental byproduct in a reality dominated by beings so ancient and powerful that our entire civilization is less significant than bacteria growing in a petri dish.

The Anthill Analogy

Consider how ants understand human civilization. They might observe our roads as mysterious flat territories, our buildings as inexplicable geometric mountains, and our daily activities as random natural phenomena. To them, we are forces of nature - incomprehensible, unpredictable, and indifferent to their existence. This is humanity's relationship to the Great Old Ones and Outer Gods of the Mythos. We build our anthill civilizations on the surface of a reality controlled by beings whose smallest actions can reshape our world, and whose very existence challenges the foundations of our sanity.

The Architecture of Cosmic Horror

The Mythos operates on multiple levels of reality, each more incomprehensible and threatening than the last. Understanding this hierarchy is crucial for both players and Keepers, as it determines the scope and scale of threats that investigators might face.

graph TD A[The Ultimate Reality] --> B[Azathoth - The Blind Idiot God] B --> C[Outer Gods - Cosmic Principles] C --> D[Great Old Ones - Ancient Rulers] D --> E[Lesser Entities - Servitor Races] E --> F[Cults and Human Servants] F --> G[Humanity - Unaware Victims] C --> C1[Nyarlathotep
Yog-Sothoth
Shub-Niggurath] D --> D1[Cthulhu
Hastur
Dagon] E --> E1[Deep Ones
Mi-Go
Shoggoths] F --> F1[Esoteric Order of Dagon
Brotherhood of the Yellow Sign
Starry Wisdom] style A fill:#000000,color:#FFFFFF style B fill:#8B0000,color:#FFFFFF style C fill:#4B0082,color:#FFFFFF style D fill:#006400,color:#FFFFFF style E fill:#8B4513,color:#FFFFFF style F fill:#B22222,color:#FFFFFF style G fill:#FFE4E1,color:#000000

The Cosmic Hierarchy Explained

Azathoth - The Nuclear Chaos

Role: The ultimate source of all existence, mindlessly creating and destroying reality

Nature: Neither good nor evil, but a blind, idiot force that dreams everything into existence while demon flautists play maddening melodies to keep it asleep

Threat level: Absolute - if Azathoth awakens, all of reality ceases to exist

Description: Azathoth exists at the center of ultimate chaos, a writhing mass of bubbling, nuclear energy from which all things emerge and to which all things return. It is the source of all creation and destruction, operating on scales so vast that galaxies are mere sparks in its cosmic dreams.

In game terms: Azathoth is more a cosmic principle than an entity investigators can meaningfully interact with. Its existence provides context for the ultimate meaninglessness that underlies Mythos horror.

The Outer Gods - Cosmic Principles

Role: Fundamental forces of reality operating on universal scales

Nature: Abstract beings that embody cosmic concepts like time, space, fertility, and knowledge

Threat level: Civilization-ending - their attention alone can reshape reality

Yog-Sothoth - The Key and the Gate

Domain: Time, space, and dimensional barriers

Manifestation: Appears as a congeries of iridescent spheres, constantly changing

Significance: Knows all things across all time and space simultaneously

Quote: "Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate."

Nyarlathotep - The Crawling Chaos

Domain: Deception, madness, and human manipulation

Manifestation: Takes many forms, often appearing human to better interact with and corrupt mortals

Significance: The only Outer God who seems to take active interest in humanity - unfortunately, this interest is malevolent

Unique trait: Often serves as the direct antagonist in scenarios, using cults and individual corruption to achieve unknowable goals

Shub-Niggurath - The Black Goat of the Woods

Domain: Fertility, life force, and biological corruption

Manifestation: A writhing mass of tentacles, mouths, and constantly spawning life

Significance: The source of many hybrid creatures and biological horrors

Cult activity: Worshipped through fertility rituals and biological experimentation

Great Old Ones - The Ancient Rulers

Role: Beings who once ruled Earth and will rule it again

Nature: Immensely powerful but not omnipotent; currently imprisoned or dormant

Threat level: Regional to global catastrophe when awakened

Game significance: These are the entities most likely to be central to major campaign arcs

The Great Old Ones: Earth's Former Masters

The Great Old Ones represent a more tangible but no less terrifying threat than the Outer Gods. These beings once ruled the Earth during prehistoric eons, before being imprisoned or forced into dormancy by cosmic events, elder seals, or their own alien lifecycles. Their eventual return is not a matter of if, but when.

Cthulhu - The High Priest of the Great Old Ones

Physical Description

Cthulhu is described as a gigantic entity with an octopus-like head, draconic body, and massive clawed hands. His most distinctive feature is the mass of writhing tentacles that surround his face like a beard. Rudimentary wings sprout from his back, though he is far too massive for normal flight. His size is deliberately vague but consistently described as mountainous - witnesses describe him as hundreds of feet tall when he briefly emerged from R'lyeh in 1925.

Current Status and Location

Cthulhu lies in a death-like sleep in the sunken city of R'lyeh, somewhere in the South Pacific. He is not truly dead, as death has no meaning for beings of his nature, but exists in a state between life and death. R'lyeh occasionally rises above the waves when "the stars are right," allowing brief periods when Cthulhu can exert influence over the world.

Prophetic Significance

Cthulhu's awakening is prophesied to herald the return of the Great Old Ones' dominion over Earth. When he rises, human civilization will end not through war or catastrophe, but through the complete irrelevance of human concerns in the face of cosmic reality.

Game Statistics (for reference - not meant to be fought!)

Sanity Loss: 1d10/1d100 to see Cthulhu

Special Abilities: Regeneration, telepathic influence, reality manipulation

Cult: Various groups worldwide, most notably the Esoteric Order of Dagon

Other Notable Great Old Ones

Hastur - The King in Yellow

Domain: Madness, artistic corruption, and forbidden performance

Manifestation: Often appears through the performance of "The King in Yellow," a play that drives audiences insane

Location: The city of Carcosa, which exists in multiple dimensions

Cult activities: Art galleries, theaters, and academic institutions where forbidden works are studied

⚠️ Special warning: Even speaking Hastur's name three times can attract his attention. Many investigators learn to refer to him only as "The King in Yellow" or "He Who Must Not Be Named."

Dagon and Hydra - The Deep Ones' Masters

Domain: The oceans and their hybrid children

Manifestation: Gigantic versions of the Deep Ones, ruling from underwater cities

Location: Various underwater cities, most notably below Innsmouth

Cult activities: Coastal communities that interbreed with Deep Ones, maritime cults

Threat: Represents the corruption of humanity through hybridization and the loss of surface world dominance

Ithaqua - The Wind-Walker

Domain: Cold, wind, and the Arctic wilderness

Manifestation: A giant humanoid figure composed of wind and ice

Location: The Arctic regions, particularly during blizzards and extreme weather

Cult activities: Inuit communities with corrupted traditions, Arctic expeditions gone wrong

Methods: Snatches victims into the sky, carrying them across vast distances before dropping them or transforming them

Yig - The Father of Serpents

Domain: Snakes, reptiles, and prehistoric knowledge

Manifestation: Usually appears as an enormous serpent or through snake-human hybrids

Location: American Southwest, areas with significant snake populations

Cult activities: Native American traditions corrupted by serpent worship, herpetological research

Curse: Known for transforming those who kill snakes into serpentine creatures themselves

Servitor Races: The Willing and Unwilling Servants

Between the cosmic entities and humanity exist various races of beings that serve the Great Old Ones and Outer Gods. Some serve willingly as ancient allies, others are created specifically for servitude, and still others are enslaved or corrupted versions of once-free species.

graph LR A[Servitor Races] --> B[Ancient Allies] A --> C[Created Servants] A --> D[Enslaved Races] A --> E[Hybrid Offspring] B --> B1[Mi-Go
Star-spawn
Flying Polyps] C --> C1[Shoggoths
Hunting Horrors
Byakhee] D --> D1[Deep Ones
Ghouls
Serpent People] E --> E1[Human-Deep One
Human-Serpent
Other Hybrids] style B fill:#4B0082 style C fill:#8B0000 style D fill:#006400 style E fill:#B8860B

Major Servitor Races

Deep Ones

Appearance: Amphibious humanoids with fish and frog-like features, webbed hands and feet, bulging eyes

Origin: Ancient race that predates humanity, possibly from another planet originally

Relationship to humanity: Seeks to interbreed with humans, creating hybrid offspring that eventually transform fully into Deep Ones

The Deep One Hybrid Transformation

This is one of the most insidious threats in the Mythos - the gradual transformation of human bloodlines:

  1. First Generation: Appears fully human but may have unusual aquatic abilities
  2. Second Generation: Subtle physical changes - slightly bulging eyes, rough skin
  3. Third Generation: More pronounced changes become apparent in middle age
  4. Full Transformation: In their 40s-50s, hybrids undergo "the change" and become full Deep Ones

Horror element: Investigators might discover they have Deep One ancestry and face an inevitable transformation

Locations: Underwater cities worldwide, most famously connected to Innsmouth

Abilities: Immortal unless killed, superhuman strength underwater, telepathic communication

Mi-Go (The Fungi from Yuggoth)

Appearance: Crustacean-like beings with fungoid characteristics, multiple appendages, and the ability to fly through space

Origin: Extraterrestrial race from Yuggoth (Pluto), highly advanced scientifically

Relationship to humanity: View humans as interesting specimens for experimentation and study

Mi-Go Activities on Earth
  • Mining operations: Extract rare minerals needed for their technology
  • Scientific research: Study human biology and consciousness
  • Brain extraction: Can remove human brains and keep them alive in cylinders for transport
  • Infiltration: Sometimes work through human agents or wear artificial human forms

Unique horror: The Mi-Go represent the fear of scientific advancement without ethics, treating humans as laboratory subjects

Locations: Remote mountain regions, underground installations, temporary bases

Technology: Advanced beyond human understanding, including space travel and consciousness transfer

Shoggoths

Appearance: Massive, amorphous blobs of protoplasmic matter covered in eyes and mouths

Origin: Artificially created by the Elder Things as multipurpose servants and laborers

Current status: Most of their creators are dead, leaving Shoggoths largely uncontrolled

The Shoggoth Rebellion

One of the most terrifying aspects of Shoggoths is their gradual development of intelligence and independence:

  • Original purpose: Mindless tools capable of forming any needed shape or structure
  • Gradual awakening: Over eons, developed basic consciousness and resentment
  • The rebellion: Eventually turned against their Elder Thing creators
  • Current threat: Free-roaming Shoggoths with increasing intelligence and hostility toward all life

Metaphor: Represents the fear of created beings surpassing and turning against their creators

Abilities: Infinite shapeshifting, incredible strength, rapid regeneration, mimicry of sounds and forms

Locations: Antarctic ruins, deep ocean trenches, underground caverns

Ghouls

Appearance: Humanoid but elongated and monstrous, with canine features and rubbery skin

Origin: Humans who underwent gradual transformation through cannibalistic practices and exposure to certain conditions

Society: Live in underground communities, often beneath cemeteries and cities

Ghoul Communities
  • Location: Extensive tunnel systems beneath major cities
  • Diet: Primarily corpses, but will kill for fresh meat when necessary
  • Intelligence: Retain human-level intelligence but with alien priorities
  • Reproduction: Can transform humans through feeding and exposure to ghoul secretions

Investigation hook: Characters might discover that missing persons are being taken underground for transformation

Relationship to surface world: Generally avoid humans but sometimes recruit or transform individuals

Special note: Some ghouls maintain enough humanity to be potential allies against greater threats

Forbidden Knowledge: The Tomes of Truth

Knowledge in the Mythos is not neutral - it is alive, infectious, and dangerous. The great tomes of Mythos lore are not simply books containing information; they are repositories of cosmic truth so alien and terrible that human minds cannot process them without fundamental alteration. Reading these books is like staring directly into the face of an incomprehensible universe.

The Great Grimoires

The Necronomicon

Author: Abdul Alhazred (the Mad Arab)

Original title: "Al Azif" (referring to the sound made by insects at night, believed to be the howling of demons)

History: Written in Damascus in 730 CE, translated into Greek as "Necronomicon" in 950 CE, later translated into Latin

Game Effects of Reading
  • Sanity Loss: 1d4/1d8 per week of study
  • Mythos Knowledge Gain: +7 Cthulhu Mythos skill
  • Spell Access: Contains numerous spells for summoning and binding entities
  • Special Knowledge: Detailed information about Great Old Ones and their servants

⚠️ Reading Warning: Complete reading typically results in permanent madness but grants deep understanding of cosmic truth

Notable Contents
  • The genealogy of the Great Old Ones
  • Rituals for contacting Outer Gods
  • Maps of other dimensions and hidden locations
  • The prophecy of Cthulhu's return
  • Instructions for creating protective wards

De Vermis Mysteriis (Mysteries of the Worm)

Author: Ludwig Prinn

Written: 1542, while Prinn was imprisoned by the Inquisition in Brussels

Focus: Primarily concerned with earthly supernatural entities and European occult traditions

Specialized Knowledge
  • Detailed information about ghouls and their society
  • European witch cults and their connections to Mythos entities
  • Necromantic rituals and communication with the dead
  • The relationship between human magic and Mythos forces

The King in Yellow

Nature: A play rather than a scholarly work

Danger: Reading the complete text drives the reader insane

Content: A two-act play that reveals cosmic truths through dramatic narrative

⚠️ EXTREME CAUTION REQUIRED

Unlike other tomes, "The King in Yellow" is designed to be psychologically contagious. Even partial reading can trigger obsessive behavior, hallucinations, and the compulsion to stage the play or share it with others.

  • First Act: Appears to be a normal, if strange, play
  • Second Act: Reveals truths about Hastur and Carcosa that shatter sanity
  • Performance: Actually staging the play can summon Hastur himself

The Pnakotic Manuscripts

Origin: Pre-human civilization, possibly written by the Great Race of Yith

Age: Hundreds of thousands of years old

Content: Earth's true history from a non-human perspective

Unique Perspectives
  • The rise and fall of pre-human civilizations
  • The war between the Elder Things and the Star-spawn
  • Detailed timelines of cosmic cycles and returnings
  • Maps of ancient cities and hidden locations
  • Prophecies written from outside linear time

Places of Power: Mythos Locations

Certain locations on Earth (and beyond) serve as focal points for Mythos activity. These places are where the barriers between dimensions are thin, where ancient entities once ruled, or where cosmic forces have left permanent marks on reality.

Sunken Cities

R'lyeh - Cthulhu's Tomb-City

Location: South Pacific Ocean, coordinates shift with cosmic cycles

Description: A city of impossible geometry that rises above the waves when "the stars are right"

Danger: Even approaching R'lyeh can drive sailors mad; landing on it during its emergences can result in direct contact with Cthulhu

Architecture: Non-Euclidean structures that hurt to perceive, made of green stone that seems to shift and flow

Y'ha-nthlei - Deep One City

Location: Beneath the waters off Innsmouth, Massachusetts

Description: Underwater metropolis of the Deep Ones, connected to the surface through underwater tunnels

Significance: Center of Deep One operations on the East Coast, where human-Deep One hybrids are taken for their final transformation

Cursed Towns

Innsmouth, Massachusetts

Surface appearance: Decaying New England fishing town

Hidden reality: Community largely composed of Deep One hybrids

History: Made a pact with the Deep Ones in the 1840s for fishing prosperity in exchange for interbreeding

Current status: Under federal investigation after the "Innsmouth Raids" of 1928

Visiting Innsmouth:

  • Local inhabitants have distinctive "Innsmouth look" - bulging eyes, scaled skin
  • Many buildings show signs of water damage despite being inland
  • Strange jewelry featuring deep-sea motifs is common
  • Visitors are watched carefully and may be followed
Dunwich, Massachusetts

Character: Isolated rural community in decline

Significance: Site of the Whateley family's attempts to summon Yog-Sothoth

Features: Ancient stone circles, the destroyed Whateley farmstead, and areas of persistent supernatural activity

Ongoing threats: Remnants of dimensional rifts, occasional creature sightings, corrupted local folklore

Ancient Ruins

The Nameless City

Location: Arabian Peninsula, hidden in the desert

Age: Predates human civilization by millions of years

Original inhabitants: Reptilian beings that worshipped ancient gods

Current status: Ruins haunted by the ghosts of its builders and guarded by supernatural forces

Dangers: Extreme heat, structural collapse, supernatural guardians, and madness-inducing hieroglyphs

The Antarctic City

Builders: The Elder Things (also called Old Ones)

Discovery: Found by the Miskatonic University Antarctic Expedition

Contents: Vast laboratory complexes, murals depicting Earth's true history, and surviving Shoggoths

Significance: Reveals that humanity is neither the first nor most important intelligent species on Earth

Using the Mythos in Your Game

The Cthulhu Mythos is a toolkit, not a rigid canon. Different Keepers emphasize different aspects depending on their group's preferences and the themes they want to explore. The key is maintaining the sense of cosmic insignificance and incomprehensible threat while still creating engaging gameplay.

Principles for Mythos Integration

Less is More

The most effective Mythos encounters hint at vast cosmic horror rather than fully revealing it. A glimpse of Cthulhu's tentacle is more terrifying than a detailed battle with the entity himself.

Example: Instead of having investigators fight a Shoggoth directly, have them discover a laboratory where one was recently contained, with evidence of its recent escape and the scientist's final, panicked notes.

Build Gradually

Start with mundane mysteries that slowly reveal supernatural elements. This mirrors the investigators' own journey from skepticism to horrified belief.

Investigation progression:

  1. Missing person case
  2. Strange witness accounts
  3. Historical patterns
  4. Supernatural evidence
  5. Mythos revelation

Make Knowledge Dangerous

Every piece of Mythos knowledge should come with a cost. Characters become more capable of understanding and fighting cosmic threats, but less capable of maintaining their humanity.

Ways knowledge becomes dangerous:

  • Sanity loss from learning terrible truths
  • Attracting unwanted attention from entities
  • Becoming unable to relate to normal people
  • Developing obsessive, self-destructive research habits

Maintain Human Scale

Even when dealing with cosmic entities, focus on human-scale consequences. How does the threat of Cthulhu's return affect specific individuals and communities?

Human-scale impacts:

  • Families torn apart by cult recruitment
  • Communities corrupted by supernatural influence
  • Individuals struggling with inherited curses or transformations
  • The personal cost of fighting incomprehensible enemies

Practice Activities

Activity One: Entity Encounter Design

Design an encounter with a Mythos entity that emphasizes horror over combat:

Entity: A lone Deep One that has come ashore

Setting: Small coastal town during a storm

Goal: Create fear and investigation opportunities without direct combat

Consider: How do the investigators first become aware of it? What evidence does it leave? How can they deal with it without fighting?

Activity Two: Forbidden Knowledge Integration

Plan how a character might discover Mythos knowledge through their profession:

Character: Dr. Sarah Mitchell, marine biologist

Discovery method: Deep sea research expedition

Challenge: How does she gradually realize what she's found?

Design progression: What does she find first? How does each discovery lead to the next? What's the final revelation?

Activity Three: Location Development

Create a Mythos location that could serve as a campaign centerpiece:

Base concept: An old mansion with a dark history

Mythos connection: Previous owner was a scholar of forbidden knowledge

Current status: Recently inherited by an investigator

Develop: What's hidden in the house? What ongoing threats exist? How does the location evolve as investigators explore it?

Activity Four: Cult Design

Design a modern cult that serves a Mythos entity:

Entity served: Hastur (The King in Yellow)

Modern context: Art gallery and theater community

Recruitment method: Through "exclusive" performances and art showings

Questions to answer: How do they identify potential recruits? What do new members experience? What's their ultimate goal?

Themes and Philosophical Implications

Cosmic Insignificance

The central theme of the Mythos is that humanity occupies no special place in the universe. We are not the crown of creation, but an accidental byproduct in a reality governed by forces that neither know nor care about our existence.

The Danger of Knowledge

Unlike most fictional universes where knowledge is power and understanding brings triumph, the Mythos presents a reality where some truths are too terrible for human minds to process safely. Curiosity becomes a tragic flaw rather than a virtue.

The Illusion of Progress

Scientific and social progress, viewed from a Mythos perspective, are temporary illusions. Human civilization is a brief flicker between cosmic ages when other beings ruled the Earth and will rule it again.

Transformation and Loss of Humanity

Many Mythos threats don't simply kill - they transform victims into something no longer human. This represents the fear of losing not just life, but identity and connection to the human community.

The Mythos and Modern Anxieties

Scientific Discovery

The Mythos reflects real anxieties about scientific advancement revealing uncomfortable truths about human nature and our place in the universe. From evolutionary theory to quantum physics, science has repeatedly challenged human-centered worldviews.

Environmental Concerns

Themes of ancient powers stirring beneath the earth and in the oceans resonate with modern environmental anxieties about awakening forces beyond human control.

Globalization and Cultural Change

The fear of transformation and loss of human identity reflects anxieties about rapid cultural change and the erosion of traditional identities in a connected world.

Information Overload

The concept of dangerous knowledge takes on new relevance in an age where access to information can be overwhelming and where some knowledge (about violence, conspiracy theories, etc.) can indeed be psychologically harmful.

Embracing the Cosmic Perspective

The Cthulhu Mythos offers both horror and strange comfort. Yes, it suggests that humanity is insignificant in cosmic terms and that terrible entities exist beyond our understanding. But it also provides a framework for confronting the genuinely frightening aspects of existence - meaninglessness, mortality, and the limits of human knowledge - in a fictional context where brave individuals can still make meaningful choices.

The investigators in Call of Cthulhu may not be able to save the world or defeat cosmic entities, but they can protect individuals, preserve knowledge for future generations, and maintain their humanity in the face of inhuman truths. Sometimes, that's enough.

Remember: the Mythos is vast and unknowable, but you don't need to understand all of it to tell compelling stories within it. Start small, build gradually, and always keep the focus on how cosmic horror affects the people at your gaming table.

"We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. But sometimes, just sometimes, the voyage is worth taking - if only to remind us how precious our small island truly is." - Adapted from H.P. Lovecraft