The Darkness is Alive. And it Hungers.
--From the Journals of Illi'thi'car, Abyss Monk

It remains unclear as to whether the Abyssal Demons are chunks of Abyssal Energy come to life, beings from other universes who use it as a road, the result of deliberate experimentation by mad Chaosians, or some combination of all of the above.

They range from nearly mindless Gremlins who randomly destroy things to cunning shapeshifters who slowly pervert and destroy entire shadows to friendly beings with bit of a temper and tendency to violence. Abyssal demons tend to be destructive, but the range of what they destroy, how they destroy, and why they destroy ranges quite a bit.

Notable kinds of Abyssal Demons include:

Some Abyssal Demons are hostile to all non-Abyssal life. Most are somewhat willing to contract with people who have something to offer; most of the Great Houses have one or more clans of demons on retainer as muscle, guardians, spies, etc.

All Abyssal demons can exist freely in the Abyss and move around with it; rarer and more prized (and deadly) are those who can open Abyss portals to the Abyss from anywhere, using it as a shortcut for rapid movement.