Refers to two things: the 1926 short story that gives the Cthulhu Mythos its titular cosmic entity, Cthulhu, and the tabletop role-playing game based on said mythos by Chaosium, often abbreviated as CoC, which has been releasing since 1981. For posts which refer to the short story specifically, tag it with the mythos tag.
In Japan, Call of Cthulhu is the predominant TRPG (table talk RPG) of choice in the 21st century, over Dungeons and Dragons which previously dominated TRPG circles there, building off the TRPG fan culture tradition of 'replays' (novelizations of sessions, often paired with visuals to go with the text) with play videos (prerecorded or live) on Niconico and YouTube (ex. Yukkuri-tachi no Cthulhu no Yobikoe). Combined with general continued popularity for 20th century western authors such as Lovecraft, an accessible and easy-to-learn 'Basic Roleplaying' system which allows players to shape the scenarios as they wish (leading to many non-combat/horror-oriented scenarios), publisher support, player-created tools (such as CCFOLIA), and domestically-exclusive scenarios, CoC dominates. In turn, this has resulted in much fanart based on Japanese scenarios.
