Old Gods are monsters that cling to the nether realms at the edge of the universe. They are among the few creatures that can survive in the Nullspace.
Inside their nightmare realm, the Old Gods slumber fitfully. Their consciousness projects into the Astral plane and the Dreamlord keeps their minds locked away behind the highest walls and deepest canyons, as part of an ancient pact and responsibility claimed long ago.
At the fall of Atlantis, Morgan le Fay used the Cosmic Cube to summon C'thulu to the city of R'lyeh. The power required to seal C'thulu in slumber broke the Demiurge and brought about the apocalypse called Ragnarok.