Cthulhu (pronounced "K'Thoo Loo") (Other spellings: Kutulu, Ktulu, Cthulu, Kthulhut, Thu Thu, Tulu) is a fictional entity (called one of the "Great Old Ones") variously described as a "sleeping" or "dreaming" in the works of H. P. Lovecraft. Cthulhu sleeps in the sunken city of R'lyeh and when the stars are right, will reawaken ushering in the destruction of humanity. It is featured in the eponymous short story The Call of Cthulhu, as well as many other works by both Lovecraft and his contemporaries.
Some have suggested a connection between Cthulhu and Judeo-Christian figures such as Satan, though this comparison is shaky at best. In every Lovecraft story, the universe is full of strange and terrible entities that humans mistakenly call Gods; Cthulhu being a powerful but not unique creature with Godlike powers. Lovecraft himself was an atheist, and his stories reflected his views of humanity's role in the universe as insignificant and at the mercy of greater, unknown powers.
In 1932, Lovecraft himself declared: "All I say is that I think it is damned unlikely that anything like a central cosmic will, a spirit world, or an eternal survival of personality exist. They are the most preposterous and unjustified of all the guesses which can be made about the universe, and I am not enough of a hair-splitter to pretend that I don't regard them as arrant and negligible moonshine. In theory I am an agnostic, but pending the appearance of radical evidence I must be classed, practically and provisionally, as an atheist."
- H.P. Lovecraft Letter to Robert E. Howard (August 16, 1932), in Selected Letters 1932-1934 (Sauk City, WI: Arkham House, 1976), p.57.