The Great Old Ones are a class of cosmic entities ranked below the Outer Gods but still vastly exceeding human comprehension. They ruled the Earth in remote antiquity and are now imprisoned, banished, or locked in deathlike slumber. Cthulhu, sleeping beneath R'lyeh, is the most famous exemplar.
Even dormant, the Great Old Ones exert influence through dreams and psychic emanations, maddening susceptible minds and inspiring cults that await their return.