The Old Ones are an ancient race of very powerful beings. They are based upon H. P. Lovecraft's Cthulhu Mythos.
The Old Ones are an ancient race of demons or gods. They once ruled the mortal world, but were later been cast out and locked away from reality. Being primal spiritual entities, they cannot ever truly be destroyed. Not all are Outside, however, as some, like the one known as the Sleeper, are on Earth, but imprisoned and entombed.
Even today, they can be still active, feeding on psychic energy, primarily the fear and awareness of them present in humanity. For that reason, the Venatori seek to wipe out knowledge, and thus influence, of the Old Ones in the Oblivion War. The existence of the Old Ones is also one of the last things wizard apprentices learn about in a traditional apprenticeship.