Along with the Nine Hells, the Abyss is the other plane of pure evil. Unlike the Nine Hells, though, the Abyss is not the home of servants of the Worlsmith. The Abyss is home to the Demons, creatures that seek to undo the Worldsmith’s work. They are not cunning. They are not trying to corrupt the races. The demons wish nothing more than the wholesale slaughter of every living thing on Crucible, and the unraveling of the fabric of the world. They are fundamentally inimical to all life, and will stop at nothing to destroy it.
Scholars have only guesses as to how the Abyss functions, believing it exists “below” even the Nine Hells. Legends say that it was created when the Fiends had begun to ignore their duty of corruption in favor of their own inscrutable goals. Enraged, the Worldsmith traveled down into the Nine Hells itself and at the bottom of the Ninth Hell itself, he cast the traitors into a deep pit, so far away from the sane world that they were driven mad.