Cthulhu was a Great Old One that Michael encountered several times.
Cthulhu descends from Yog-Sothoth, possibly having been born on Vhoorl, in the 23rd nebula. He mated with Idh-yaa on the planet Xoth. His offspring are Ghatanothoa, Ythogtha, Zoth-Ommog, and Cthylla.
He also is said to have mated with the being Nyarlathotep, producing Ilyth'la.
Cthlhu arrived on Earth with his Star-Spawn, the Star-Spawn warred with the Elder Things, the details of the war are unknown, but the result of the war was that Cthulhu's was entombed in a stone crypt deep within the planet Earth in the city Ry’leh . Its separated consciousness travelled along the time winds while its sleeping body retained its subconscious mind. The two could not be merged until an alignment of the stars as they had been when Cthulhu was whole. Due to the universe's constant expansion and evolution, perfect alignment would be impossible and Cthulhu needed others to perform rituals that would give the subconscious additional focus in drawing the consciousness from cosmic forces.
When the correct alignment occurred in1915, Cthulhu's body was able to dream for several days and this created a pervasive psychic background at .338 microbars of pressure. Gilles, using the voodoo ritual of Wete Mo Nan Dlo, attempted to restore Cthulhu. Cthulhu, in turn, helped Mait in his plans to create an army of zombies and a device to control them. Before Mait could awaken Cthulhu, Michael managed to set explosives in the tomb which killed Mait and resealed the Great Old One's tomb.
In 1933, Cthulhu attempted to rise again with the help of Dagon and his cult, however, Michael stopped the cult when he killed Dagon and destroyed the cult base. Cthulhu was reawakened for a time. However, Michael grew to giant size and battled Cthulhu until he was defeated.