Lol, he's a monster from a book called, the Call of Cthulhu. It was written by H.P. Lovecraft in the 1800's.
In the story , he's one of The Great Old Ones, a monster from the skies that was trapped in the ocean, and there he slumbers. Legend has it that should he awaken he will destroy the world. He also visits his followers in their dreams, giving them orders to do his bidding, so that he may awaken.
Also, from Wiki:
"Cthulhu is often referred to as a tongue-in-cheek shorthand for extreme horror or evil."