A Horror background or Shared World initially elaborated by H P Lovecraft and his circle, including Robert Bloch, August Derleth, Robert E Howard, Frank Belknap Long and Clark Ashton Smith – hence such recursive (> Recursive Fantasy) Lovecraftian jokes as the ascription of a grimoire to the Comte d'Erlette (Derleth) and the driving to insanity of "Robert Blake" (Bloch) in "The Haunter of the Dark" – the latter in mock retaliation for Bloch's Lovecraft pastiche "The Shambler from the Stars" (1935 WT).
The CM centres on a pantheon of monstrous "Great Old Ones" whose mere appearance may cause insanity; these, as systematized by Derleth from Lovecraft's shuddering hints, include the Malign Sleeper Cthulhu, the Guardian of the Threshold Yog-Sothoth, the obscenely parodic Fertility deity Shub-Niggurath, and their Hermes-messenger Nyarlathotep. The essence of Lovecraft's "cosmic terror" is these entities' unconcern for such small fry as humanity in a vast Universe: foolish enquirers are withered or destroyed almost incidentally. Ancillary features are: a recurring set of accursed grimoires such as the Necronomicon (> Books); various hideous (even when semi-benign) Elder Races, not to be confused with the Elder Gods; lesser horrors like shoggoths, night-gaunts and Hounds of Tindalos; New England's haunted town Arkham (home of Miskatonic University) and decayed seaport Innsmouth; and other mystically charged names – Carcosa, Hali, Hastur – borrowed from Ambrose Bierce via Robert W Chambers's The King in Yellow (coll 1895).
Lin Carter's Lovecraft: A Look Behind the Cthulhu Mythos (1972) examines the CM in exhaustive detail and includes a bibliography, as does The Reader's Guide to the Cthulhu Mythos (1969; rev 1973) by Robert E Weinberg and Edward P Berglund. The retrospective Tales of the Cthulhu Mythos (anth 1969; 1975 2 vols UK) ed Derleth assembles early stories; new contributions appear in The Disciples of Cthulhu (anth 1976) ed Berglund, New Tales of the Cthulhu Mythos (anth 1980) ed Campbell, Shadows Over Innsmouth (anth 1994) ed Stephen Jones – a theme anthology inspired by Lovecraft's "The Shadow Over Innsmouth" (included in the volume) and its sea-dwelling Deep Ones with which human cultists miscegenate – and Cthulhu 2000: A Lovecraftian Anthology (anth 1995) ed Jim Turner.
The Game Call of Cthulhu unwisely quantifies the powers and traits of CM unknowables, leading to such mildly silly spinoffs as Petersen's Field Guide to Cthulhu Monsters * (graph 1988) by Sandy Petersen, Tom Sullivan and Lynn Willis. [DRL]