Cthulhu is a monstrous entity who lies "dead but dreaming" in the city of R'lyeh, a place of non-Euclidean madness presently (and mercifully) sunken below the depths of the Pacific Ocean. Cthulhu appears in various monstrous and demonic forms in early myths of the human race. Racial memory preserves Him as humanity's most basic nightmare. Cthulhu is the high priest of the Great Old Ones, unnatural alien beings who ruled the Earth before humanity formed, worshipped as gods by some misguided people. It is said that They will return, causing worldwide insanity and mindless violence before finally displacing humanity forever. 
Cthulhu is one of H.P. Lovecraft's most popular and frightening characters. Fortunately Cthulhu isn't real.