The Fall of Moldar- Orc Campaign
Demons are creatures of pure evil- existing only to inflict pain and suffering on the bodies and souls of everything that can feel it. They are formed from the twisted and tortured souls of evil individuals, and those whose souls they have stolen. Whenever a demon kills a mortal, certain types of demons can steal their soul and take it to the Abyss. The Abyss is their domain- a place of endless darkness outside the Material Plane. It is here that millions upon millions of demonic beasts dwell, teeming in endless war with each other. There is no order, law or structure in the Abyss- only that which the strongest and most powerful demons can impose on the weaker. In this way, gangs and hordes of demons battle for their own amusement and glory to temporarily rule portions of the boundless void in which they dwell.
No demon may be permanently killed inside the Abyss- the soul energy that created it simply reforms into a new body and mind, and in this way the demonic hordes can war endlessly and fruitlessly for all eternity without bleeding themselves dry.
Individually- demons are sadistic, capricious, cruel, callous, and murderous, taking delight in torturing anything that can be tortured. Many, however, are not stupid. While the lower denizens of the Abyss are quite simple, many of the more powerful ones are as intelligent as any mortal, or moreso. They can be liars or tricksters, granting wishes or making bargains that they have no intention of keeping, telling lies with seductive tongues and leading mortals into darkness and madness with their falsehoods. The variety of evil in the Abyss is as endless and as varied as the Abyss itself.
Demons are the creations of the mad god Malakai, formed from the souls of innocent and unwilling mortals that he stole on their way to the beyond. He used these souls to empower himself, and the burning fire of these souls drove him to omnicidal madness. Stealing more souls, he created hordes of demons, visions of his madness and malicious intent. He created them for one purpose- to destroy all life on Rega, and this end they accomplished with glee. With his single mind and will driving them, they fought as one entity- without the chaos and disorder of their current state. When Malakai was destroyed and his mind broken, this driving will vanished, and the demons were left to their own devices. Seeing the armies of organised Celestials and Devils opposing them and no mortals to torment, they fought only half-heartedly as they were banished to the Abyss by the combined might of the gods and their armies
Re-Discovery of Life
Ever since their banishing to the Abyss, demons have fought and tortured each other. When they discovered that mortal life had returned to Rega, however, many grew discontented with this existence. They wanted nothing more than to taste mortal suffering and dine on mortal flesh. The powerful Demon Lords pushed slightly into the Material Plane, tainting sentient minds, and soon cults of the deranged and the mad were forming to worship the lords of the Abyss and summon lesser demons into the mortal realm. While no full-scale invasion of the Material Plane has ever occurred- smaller invasions, attacks, and the predations of individual demons have brought suffering to Rega and its mortal inhabitants.
Demon Lords are the most powerful and mighty of all demonkind, and indeed are unique from all their lesser cohorts. When Malakai's power broke and scattered, much of it drifted over his powerful lieutenants, and they were strong enough to seize it for themselves. This transformed them, turning them into mighty avatars of chaos and death- the Demon Lords. There are untold dozens of these creatures, many of which are worshipped as gods by various savage, primitive tribes and crude cults. Sometimes, they have even been summoned by these foolish cults and ity has taken an immense effort in lives, magic and men to drive them back to the Abyss. But you cannot kill a Demon Lord- for it merely returns to the Abyss and re-incarnates again and again.