Cthulhu is the most famous figure of the Cthulhu Mythos spawned by horror writer H.P. Lovecraft. He made his first and only appearance in the 1928 short story "Call of Cthulhu," the definitive cosmic horror story. He isn't that important within Lovecraft's universe, a relatively minor member of its complex pantheon - the real head honcho of the cosmos is the Blind Idiot God, Azathoth, who writhes and gibbers in the nuclear chaos of creation, and Cthulhu himself is pretty much "just" a very powerful alien sorcerer-priest and king of his own species - yet Cthulhu's alien but semi-pronounceable name, the popularity and quality of "Call of Cthulhu," his frequent references in Lovecraft and his friends and follower's stories, and his iconic winged squid-man appearance have cemented his role as the poster-child of the eponymous mythos. He has since gone on to star in the roleplaying game of the same name, as well as another based on the Mythos known as Trail of Cthulhu.
He is very strong against Investigators, and can eat 1d3 of them per round; however, he is weak against boats, especially when they make called shots to the face (and are manned by Norwegians), though his fast healing usually makes up for it.