While I’ve been playing Call of Cthulhu for nearly a decade, I only recently made the jump to the online community, coinciding with my start GMing games. One thing that surprised me was the high volume of Cthulhu RPG variants and systems, only a few of which I’d previously been aware of. I did some looking around and couldn’t find a good guide differentiating between the different systems—so, inspired by klecser’s post, I decided to do the research to make one. Some of this is cribbed from Jalor218 on Reddit and Morgan on 21st Century Philosopher. Jalor218’s comment that he hadn’t played half the systems he summarized gave me the confidence to take a stab at something similar.
I've drawn attention to the most high-profile systems—the ones that I've seen referenced time and again online—by putting them in green. If I’ve misrepresented something or left out essential details, please let me know and I’ll be happy to revise this post. Additionally, if there's a prominent system I haven't included, let me know and I'll revise to add it.
I. Classic Call of Cthulhu and its near-variants:
Call of Cthulhu – Utilizes a d100 percentile dice system called the Basic Roleplaying System, or BRP. Scenarios are usually set in the 1920s. Call of Cthulhu is currently on its 7th Edition, though the rules are very similar and scenarios can be easily converted across editions. Call of Cthulhu focuses on investigation, leaves plenty of room for failure, and has lethal combat. Characters will usually die or go insane. The gamemaster is called the keeper and characters are called investigators.
Essential rulebook(s): Call of Cthulhu Keeper Rulebook, 7th Edition
Free resource: Call of Cthulhu 7th Edition Quick-Start Rules
There are several Call of Cthulhu modules that share an identical ruleset and gaming system to the usual Call of Cthulhu, but have special names to denote a special setting and a unique reference book to assist with roleplaying in that setting. These are:
Cthulhu Invictus – Set in Ancient Rome (circa 100 AD).
Essential rulebook(s): Cthulhu Invictus
Cthulhu Dark Ages – Set during the Dark Ages (circa 1000 AD).
Essential rulebook(s): Cthulhu Dark Ages
Down Darker Trails – Set in the American Old West (circa late 1800s).
Essential rulebook(s): Down Darker Trails
Cthulhu by Gaslight – Set in 1890s Victorian England.
Essential rulebook(s): Cthulhu by Gaslight
Achtung! Cthulhu – Set during World War II, with investigators playing Allied agents fighting the Secret War against the Nazi Black Sun.
Essential rulebook(s): Achtung! Cthulhu: Keeper's Guide to the Secret War
Cthulhu Now – Set in modern times.
Essential rulebook(s): Cthulhu Now
H.P. Lovecraft's Dreamlands – Set in H.P. Lovecraft's Dreamlands, where investigators travel down the seven hundred steps, through the Gates of Deeper Slumber, and into the realm of dreams.
Essential rulebook(s): H.P. Lovecraft's Dreamlands
Pulp Cthulhu – Utilizes a d100 BRP system similar to Call of Cthulhu, but with several unique rules. Scenarios are usually set in the 1930s. Pulp Cthulhu changes the game to turn the investigators into action heroes, similar to Indiana Jones, who are much less likely to die in combat and more equipped to fight the Cthulhu mythos directly. It has a pulpy, action/adventure tone.
Essential rulebook(s): Pulp Cthulhu
Trail of Cthulhu – Utilizes a mostly diceless points-based system (some limited use of d6) called the GUMSHOE System. Scenarios are usually set in the 1930s. The GUMSHOE System is designed for investigative roleplaying such that a failed dice roll cannot prevent the finding of a clue. Trail of Cthulhu emphasizes storytelling and interpreting clues, as opposed to roleplaying and gathering clues.
Essential rulebook(s): Trail of Cthulhu rulebook
Free resource: Trail of Cthulhu downloads and resources
Cthulhu Confidential – Trail of Cthulhu designed for play with one gamemaster and one player.
Essential rulebook(s): Cthulhu Confidential
Cthulhu City – Trail of Cthulhu set in a New England overrun by the Cthulhu Mythos.
Essential rulebook(s): Cthulhu City
Delta Green – Utilizes a d100 BRP system similar to Call of Cthulhu, but with several unique rules. Scenarios are usually set in the 1990s or modern times. Delta Green presents a scenario in which characters are government agents investigating and combatting mythos elements, similar to The X-Files. It takes the investigative tone of Call of Cthulhu and adds an air of conspiracy and secret societies. The gamemaster is called the handler and characters are called agents.
Essential rulebook(s): Delta Green: Agent’s Handbook
Free resource: Delta Green: Need to Know quick-start rulebook
The Fall of Delta Green – A Delta Green prequel that utilizes the mostly points-based GUMESHOE System used in Trail of Cthulhu. Scenarios are usually set in the 1960s.
Essential rulebook(s): The Fall of Delta Green
Dark Streets – Utilizes a d100 percentile dice system from Renaissance Deluxe, which takes its system from OpenQuest, which is based on BRP. Scenarios are set in 1750s London. Characters are members of London’s proto-police force. Dark Streets gameplay is very similar to traditional Call of Cthulhu, emphasizing investigation and deduction while keeping combat occasional and deadly.
Essential rulebook(s): Dark Streets 2nd Edition - Core Rulebook
Raiders of R’lyeh – Utilizes a d100 percentile dice system based on RuneQuest, a BRP system. Scenarios are set in 1900-1913. Characters are mercenary rogues exploring the frontiers of the known world. Closer to Pulp Cthulhu than traditional Call of Cthulhu, Raiders of R'lyeh looks to Robert E. Howard’s Mythos and Weird stories as an additional influence.
Essential rulebook(s): Raiders of R’lyeh: Gamemaster’s Guide & Complete Rules
GORE – Utilizes a d100 percentile dice system that is fairly rules-light and based on OpenQuest, which is based on BRP. GORE stands for Generic Old-school Roleplaying Engine, and is designed to allow Lovecraftian adventures in science fiction, fantasy, investigative, and classical horror settings.
Essential & free rulebook: GORE
II. Variants with a strong “rules-light” emphasis:
Cthulhu Dark – Utilizes a d6, rules-light system. Loosely related to Trail of Cthulhu, with an emphasis on storytelling and the how of things rather than the whether of things. Maintains a focus on cosmic horror within a barebones rules environment.
Essential rulebook(s): Cthulhu Dark hardcover rulebook
Free resource: Cthulhu Dark: A Rules-Light System for Lovecraftian Horror
Cthulhu Grey – Utilizes a d6, rules-light system closely based on Cthulhu Dark. Cthulhu Grey adds in a Harm stat, greater character differentiation, and rules for Magick.
Essential & free rulebook: Cthulhu Grey: A Cthulhu Dark Hack
Cthulhu Grim – Utilizes a d6, rules-light system closely based on Cthulhu Dark and Cthulhu Grey. Cthulhu Grim adopts rules specifically suited for campaign play.
Essential & free rulebook: Cthulhu Grim, based on Cthulhu Dark and Cthulhu Grey
Lovecraftesque – Utilizes a diceless, GMless system. Creates a GMless story of a lone character confronting cosmic horror in the form of a mystery that builds to a climax. Each playthrough will create a single one-shot scenario designed for completion in a single session. Players take on the roles of Narrator, Witness, and Watchers.
Essential rulebook(s): Lovecraftesque rulebook
Free resource: Lovecraftesque free references and handouts
De Profundis – Utilizes a diceless, GMless, correspondence-based system. Players take turns writing letters in-character to one another in the style of H.P. Lovecraft. Includes an option for solo play.
Essential rulebook(s): De Profundis Second Edition
The Cthulhu Hack – Utilizes dice of multiple sizes in a rules-light system called The Black Hack. As a rules-light system, The Cthulhu Hack emphasizes storytelling and offers player-focused gameplay designed for quick and easy pick-up play.
Essential rulebook(s): The Cthulhu Hack rulebook
Free-ish (PWYW) resource: The Cthulhu Hack: Quickstart
tremulus – Utilizes a d6 system that is fairly rules-light called Powered by the Apocalypse. tremulus is a storytelling-first game that requires limited prep and emphasizes investigation and survival.
Essential rulebook(s): tremulus rulebook
Cthulhu Abides – Utilizes a d6 “poker dice” system that is fairly rules-light and similar to a system called The Whispering Vault. Scenarios are generally set in 1920s Britain. Cthulhu Abides is designed to be heavier than Cthulhu Dark, with greater character differentiation, while still lighter than Call of Cthulhu and Trail of Cthulhu. Intended as a Pick Up & Play RPG.
Essential rulebook(s): Cthulhu Abides: A Roleplaying Game of Investigation & Madness
Free resource: Cthulhu Abides: A Roleplaying Game of Investigation & Madness (PDF)
Unnamable – Utilizes a d6, rules-light system. Unnamable greatly simplifies skill checks so that character descriptors, personality traits, and descriptions rather than ranked skills are used for skill checks. It also preserves character sanity to a greater degree than Cthulhu Dark.
Essential & free rulebook: Unnamable: A Little Lovecraft Game
Unspeakable – Utilizes a d6, rules-light system closely based on Unnamable. Unspeakable adds a bit more character differentiation as well as increases the risk of character insanity.
Essential rulebook(s): Curse of the Yellow Sign Collected
III. Adaptations intended to bring Cthulhu roleplaying into other RPG universes:
Call of Cthulhu d20 – Utilizes the d20-based system of Dungeons & Dragons, 3.5 edition. It was created to maintain the feel of the original Call of Cthulhu, but using a dice system that would be more familiar to Dungeons & Dragons players and which could allow for the porting of characters, monsters, and other content between the two universes. Currently out of print.
Essential rulebook(s): Call of Cthulhu d20 edition rulebook
Sandy Petersen's Cthulhu Mythos for 5E – Utilizes the d20-based system of Dungeons & Dragons, 5th edition. Sandy Peterson's Cthulhu Mythos is designed to allow Cthulhu roleplaying within the most recent implementation of the Dungeons & Dragons universe and system.
Essential rulebook(s): Sandy Petersen's Cthulhu Mythos (5E) + D&D Dungeon Master's Guide + D&D Monster Manual + D&D Player's Handbook
Sandy Petersen’s Cthulhu Mythos for Pathfinder – Utilizes the d20-based system of Pathfinder. Sandy Petersen’s Cthulhu Mythos is designed to allow Cthulhu roleplaying within the Pathfinder universe and system.
Essential rulebook(s): Sandy Petersen’s Cthulhu Mythos (PF) + Pathfinder Roleplaying Game Core Rulebook
World of Cthulhu – Utilizes the d10 system of World of Darkness called the Storyteller System. World of Cthulhu is designed to allow Cthulhu roleplaying within the World of Darkness universe and system.
Essential rulebook(s): World of Cthulhu: Lovecraftian Roleplay in the World of Darkness (free) + New World of Darkness Rulebook (1st edition)
Cthulhu Pulp: Tales from Beyond Pulp – Utilizes the d20 system of Pulp Adventure. Designed to allow Cthulhu roleplaying within the Pulp Adventure system, with a focus on using super-science and muscle to fight the forces of the Cthulhu Mythos. Out of print.
Essential rulebook(s): Pulp Adventure, Second Edition: Cthulhu Pulp: Tales from Beyond Pulp
IV. Other variant systems:
Cthulhu Live – Utilizes a points-based live action roleplaying (LARP) system with no randomizer. Aims to stay true to the feel of Call of Cthulhu, where investigators are flesh-and-blood humans and violence is consequential.
Essential rulebook(s): Cthulhu Live 3rd Edition (LARP)
Realms of Cthulhu – Utilizes dice of multiple sizes in a fairly rules-light system called Savage Worlds. Realms of Cthulhu remains investigative in nature but incorporates a faster pace and more action, along with the addition of Combat Magic.
Essential rulebook(s): Realms of Cthulhu rulebook + Savage Worlds Adventure Edition
Shadows of Cthulhu – Utilizes a d20 system called True20. Scenarios are usually set in the 1920s. Shadows of Cthulhu presents an experience similar to Call of Cthulhu d20, but with a higher degree of action/pulp and while utilizing the True20 system, intended to provide a d20-based system that requires no other kinds of dice and minimizes the need for tracking things like hit points.
Essential rulebook(s): Shadows of Cthulhu + True20 Adventure Roleplaying, Revised Edition
Nemesis– Utilizes a d10 dice-pool system called One-Roll Engine. Nemesis focuses on dark horror inspired by Lovecraftian themes and utilizes a Madness Meter to simulate insanity and mental trauma.
Free & essential rulebook: Nemesis: Roleplaying in Worlds of Horror
V. Cthulhu-adjacent games with their own unique tone:
Cthulhu Risus – Utilizes a d6, rules-light system called Risus. Cthulhu Risus offers a fast-paced and humorous take on Lovecraftian horror.
Essential & free rulebook: Cthulhu Risus
CthulhuTech – Utilizes a d10 dice pool “poker dice” system called Framewerk. Scenarios are set during the Aeon War in 2085. CthulhuTech blends Lovecraftian horror with anime-style mecha, sci-fi, and magic. For mature audiences only.
Essential rulebook(s): CthulhuTech Core Book
Free resource: CthulhuTech V2: The Shadow War Open Beta
Cthulhu for President – Utilizes a d6 system called the BEER Engine. Cthulhu for President offers tongue-in-cheek humor as players take on the role of Elder Party staffers in an attempt to get their Great Old One elected.
Essential rulebook(s): Cthulhu for President: The Game
Fantasy Cthulhu – Utilizes the custom dice (d6) system of FATE. Fantasy Cthulhu places the Cthulhu mythos within a fantasy setting powered by the FATE system.
Essential rulebook(s): Fantasy Cthulhu powered by FATE Core
Post-Cthulhu – Utilizes a custom dice (d6) system called FATE. Scenarios are set in the future, after Cthulhu has risen. Players take on the role of survivors of the Cataclysm who wander a post-apocalyptic landscape stalked by Lovecraftian monsters.
Essential rulebook(s): Post-Cthulhu
FATE of Cthulhu – Utilizes a custom dice (d6) system called FATE. Scenarios are set in the present day, with characters who are time-travelers from a dark future in which the Great Old Ones have returned. FATE of Cthulhu sets Lovecraftian horror against a Terminator-like backdrop. Currently on Kickstarter.
Essential rulebook(s): On Kickstarter now