Cthulhu is a fictional character that first appeared in the short story "The Call of Cthulhu", published in the pulp magazine Weird Tales in 1928. The character was created by writer H. P. Lovecraft.

The character is a central figure in Lovecraft literature, with the short story The Dunwich Horror (1928) mentioning Cthulhu, while The Whisperer in Darkness (1930) hints at the character's origins ("I learned whence Cthulhu first came, and why half the great temporary stars of history had flared forth."). The 1931 novella At the Mountains of Madness refers to the "star-spawn of Cthulhu", who warred with another race called the Old Ones before the dawn of man.

Source: Wikipedia, "Cthulhu", available under the CC-BY-SA License.