Cthulhu (alternate spellings / names: Tulu, Cthulu and many others) is a fictional character, one of a pantheon of fictional Great Old Ones of the Cthulhu Mythos in the writings of H. P. Lovecraft. "Cthulhu" is pronounced "kuh-THOO-loo", but according to Lovecraft, this is an attempt by human speakers to pronounce a name that is actually in an ancient language alien to them. Lovecraft speculated that KHLUL'HLOO, might be closer to the original name. Cthulhu is an evil godlike being who would supposedly lay waste to the world if ever awoken from his slumber in the sunken city of R'lyeh, located somewhere under the South Pacific.
Physically, Cthulhu was said to have a bloated, humanoid body, greenish in color. His head was tentacled, with the tentacles arranged about his maw. Two bat-like wings sprouted from his back. His skin was said to be "flabby" and slimy. Cthulhu's exact height is not given, but the creature was able to pursue a ship through the Pacific Ocean, albeit on some still-underwater portion of risen R'lyeh, for some distance and still keep most of its body above water. Writes Lovecraft:
Although the cycle of stories written by Lovecraft and continued by his friends and literary successors bear the badge "The Cthulhu Mythos" (a label first used by August Derleth), Cthulhu is actually one of the least of the terrible creatures in the pantheon and indeed makes only one appearance (The Call of Cthulhu) as an active character in all of Lovecraft's work.
According to Derleth's stories, Cthulhu is neither the most powerful one nor a leader among the god-like mythos creatures; those would be Yog-Sothoth, "the Lurker at the Threshold", and Azathoth, "the blind, idiot god", respectively. Like many others, Cthulhu is worshipped by a cult of followers that manages to survive through the centuries, and his cult is indeed the most wide-spread, containing the largest number of worshippers and arguably the most effective at recruiting new members. Though Cthulhu himself continues to be sleeping and dreaming in R'lyeh, this cult continues to expand its activities in his name. The cult's behind-the-scenes actions play a major role in stories of the Mythos, and through them Cthulhu continues to have an indirect influence in many stories.
Though arguably the best known figure of the pantheon, a number of stories by Derleth present his alliance with the other Mythos creatures as an uneasy one at best.
At least one of the other Great Old Ones acts as a rival to his power and a personal enemy to Cthulhu. This is Hastur, "Lord of the Interstellar Spaces," currently reciding in the Hyades. Derleth did not create Hastur, but he was responsible for his introduction to the Mythos as Cthulhu's worst enemy and half-brother. Various stories present Hastur's cult offering assistance to those trying to prevent Cthulhu's awakening, or make other mention to the rivalry between the two. In Darleth's The Return of Hastur, first published in March, 1939, the two Gods even physically confront each other, albeit briefly.
Cthulhu has several avatars: