Cthulhu is one of the Great Old Ones (a.k.a. Elder Gods) in the Cthulhu Mythos, worshiped by the Cult of Cthulhu, and first mentioned in the Necronomicon by Abdul Alhazred. His existence was brought to light by writer H. P. Lovecraft, in the short story "The Call of Cthulhu".
Nowadays he is one of many Elder Gods recognized by the Church of the SubGenius, alongside others such as Jehovah 1 and Eris Discordia, and is also worshiped by the Cult of Cthulhu to this day. Cthulhu lives in the sunken city of R'lyeh, which is in another dimension, and the portal into it is in the South Pacific somewhere in the area near New Zealand. He has a squid-like appearance, and the prairie squid that are commonly found by SubGenii and kept as pets are actually descendants of Cthulhu that originally sprang forth from him. He is known to the Pastafarians as the Flying Spaghetti Monster, although they are quite misguided about his true nature and are simply tools in his plans to return and cause havoc and destruction.
Cthulhu iT the most evil being of all, infinitely more evil than that wimp Satan. Here is an excellent classic-style badfilm called The Call of Cthulhu, which is unfortunately on Hulu rather than YouTube, so you'll see advertisements before you can watch it: