Abyss/Thoon

The Chaos From Outside

The Abyss was not originally a place, but a person.
When the world was new, before the New Gods were even born, the Old Gods were attacked by an invader - a massive god from outside of the Arcane Realms, that nearly destroyed them. The old gods fought desperately, and finally won, but at a cost. Keryth realized that to destroy the Abyss, she must eat it. And then, when she began to show possible signs of corruption by the Abyss, Vutualli/Rhother/Bogtin Sundered her.
The Abyss is dead (mostly), but it still corrupts. Each of the Old Gods carries traces of its corruption, and its corpse became the Abyss, which corrupts most creatures that live there - even Vutualli/Rhother/Bogtin, exiled there for the crime of killing another god. Warlocks of the Great Old One tap into the corrupting power of the Abyss, potentially risking the destruction of all.
Unbeknownst to the gods, however, the Abyss has multiple aspects. One of them is Thoon, who had a similar war with the gods of the Psionic Realms and who has been locked away, quietly corrupting members of the Illithid Empire into the cultists named for it.