Cthulhu is a god who figures very largely in Lovecraft's Cthulhu mythos. This tentacle-faced denizen of the sea lies sleeping in R'Lyeh where he waits for the time when his followers will release him from his chains so he can rule the world once more. He is an evil god who is bent on the destruction of the human race, despite this fact he does have a large number of followers who do actively try to release him using such books as the Necronomicon.
Cthulhu can be found in the story "The Call of Cthulhu" by H.P. Lovecraft.