Said to have been created when the Nameless planted a seed or shard of pure evil in the depths of the Elemental Chaos during the Dawn War, the dark energies of the seed seemed in warp the Primordials that dwelled there as well, making them more chaotic and evil than before. The demons of the Abyss seek only to destroy and ravage. They especially desire to vent their rage and hated on Tol.
The most powerful demonic denizens of the Abyss are referred to as Princes, although they possess no true hierarchy other than a sort of ranking by sheer power. If a demon is strong enough to declare itself a Prince, and no one is strong enough to stop it, then it is considered a Demon Prince.
Theoretically, the Abyss is an infinite subsection of the infinity of the Elemental Chaos. Popular fancy says that there are 666 sub-realms within the Abyss, but this is likely nothing but legend and superstition.