Call of Cthulhu is a horror fiction role-playing game based on H. P. Lovecraft's story of the same name and the associated Cthulhu Mythos. The game, often abbreviated as CoC, is published by Chaosium; it was first released in 1981 and is in its seventh edition, with licensed foreign language editions available as well. Its game system is based on Chaosium's Basic Role-Playing (BRP) with additions for the horror genre. These include special rules for sanity and luck.
Call of Cthulhu is set in a darker version of our world based on H. P. Lovecraft's observation (from his essay, "Supernatural Horror in Literature") that "The oldest and strongest emotion of mankind is fear, and the strongest kind of fear is fear of the unknown." The original edition, first published in 1981, uses Basic Role-Playing as its basis and is set in the 1920s, the setting of many of Lovecraft's stories. The Cthulhu by Gaslight supplement blends the occult and Holmesian mystery and is mostly set in England during the 1890s. Cthulhu Now and Delta Green are set in a modern/1980s era and deal with conspiracies. Recent settings include 1000 AD (Cthulhu: Dark Ages), the 23rd century (Cthulhu Rising) and Ancient Rome (Cthulhu Invictus). The protagonists may also travel to places that are not of this earth, such as the Dreamlands (which can be accessed through dreams as well as being physically connected to the earth), other planets, or the voids of space. In keeping with the Lovecraftian theme, the gamemaster is called the Keeper of Arcane Lore ("the keeper"), while player characters are called Investigators of the Unknown ("investigators").
While predominantly focused on Lovecraftian fiction and horror, playing in the Cthulhu Mythos is not required. The system also includes ideas for non-Lovecraft games, such as using folk horror or the settings of other authors and horror movies, or with entirely custom settings and creatures by the gamemaster and/or players.
CoC uses the Basic Role-Playing system first developed for RuneQuest and used in other Chaosium games. It is skill-based, with player characters getting better with their skills by succeeding at using them for as long as they stay functionally healthy and sane. They do not, however, gain hit points and do not become significantly harder to kill. The game does not use levels.
CoC uses percentile dice (with a results ranging from 1 to 100) to determine success or failure. Every player statistic is intended to be compatible with the notion that there is a probability of success for a particular action given what the player is capable of doing. For example, an artist may have a 75% chance of being able to draw something (represented by having 75 in Art skill), and thus rolling a number under 75 would yield a success. Rolling 1⁄5 or less of the skill level (1-15 in the example) would be a "special success" (or an "impale" for combat skills) and would yield some extra bonus to be determined by the keeper. For example, the artist character might draw especially well or especially fast, or catch some unapparent detail in the drawing.
The players take the roles of ordinary people drawn into the realm of the mysterious: detectives, criminals, scholars, artists, war veterans, etc. Often, happenings begin innocently enough, until more and more of the workings behind the scenes are revealed. As the characters learn more of the true horrors of the world and the irrelevance of humanity, their sanity (represented by "Sanity Points", abbreviated SAN) inevitably withers away. The game includes a mechanism for determining how damaged a character's sanity is at any given point; encountering the horrific beings usually triggers a loss of SAN points. To gain the tools they need to defeat the horrors – mystic knowledge and magic – the characters may end up losing some of their sanity, though other means such as pure firepower or simply outsmarting one's opponents also exist. CoC has a reputation as a game in which it is quite common for a player character to die in gruesome circumstances or end up in a mental institution. Eventual triumph of the players is not guaranteed.