When the Morrigan claimed dominion over several layers of the Abyss eons ago, she transformed her entire host of angels into war demons — but there were already demon lords lurking in the corners of the plane, born from dead gods, primordials, and nightmares. The energies of the Abyss corrupt all that remains there, corrupting anything that comes into contact with the Abyss into a demonic form. The original source of this demonic energy remains unknown.
The uncounted layers of the Abyss shift and slide over one another in a roiling mass of evil. Ruling over these misshapen layers are the demon lords, fiends whose power resembles that of a minor deity. If they worked together, they could easily overrun the cosmos, but their chaotic nature keeps them trapped at each other’s throats, sending billions of demons to fight, die, and be reborn every day.
Demons rarely intend to cross into the mortal world, and arrive either by accident or by the intent of a mortal summoner or a demon lord. Once there, demons run amok, razing and destroying everything in their path with all of the magic and weaponry at their disposal. As demons can only be controlled by the most rigorous magic, summoning a demon into the world is a quick way to get yourself, and everything around you, killed horribly.