The great old ones are an extremly old race of cosmic beings all trillions of years old. That rebelled against the Elder Gods, both the EGs and GOOs were immortal, so the EGs sealed both the GOOs and themselves in the far reaches of space so the universe could be saved. The Goos were imprisoned on Earth. The EGs in the sun. Cthulhu is considered to be the most powerful, due to the fact that he can manipulate people to do his bidding.