Demon

'Demon are violence and chaos incarnate, they exist to destroy and often take glee in it '- Perr Dithine

Spawned in the Infinite Layers of the Abyss, demons are the embodiment of chaos and evil. They are engines of destruction barely contained in monstrous form. Possessing no compassion, empathy, or mercy, they exist only to destroy.
The Abyss creates demons as extensions of itself, spontaneously spawning fiends from the aether to bring about carnage. Some are unique monstrosities, while others are common in their form. Other demons (such as manes) are created from mortal souls shunned or cursed by deities, or which are otherwise trapped in the Abyss.
Demons respect power and power alone. Demon lord and greater demons command shrieking mobs of lesser demons because it can destroy any lesser demon that dares to refuse its commands. A demon's status grows with the blood it spills; the more enemies that fall before it, the greater it becomes.
A demon might spawn as a manes, then become a dretch, and eventually transform to a vrock after untold time spent fighting and surviving in the Abyss. Such elevations are rare, however, for most demons are destroyed before they attain significant power. The greatest of those that do survive make up the ranks of the Demon Lords that threaten to tear the Abyss apart with their endless warring.
By expending considerable magical power, Demon Lords can raise lesser demons into greater forms, though such promotions never stem from a demon's deeds or accomplishments. Rather, a demon lord might warp a manes into a quasit when it needs an invisible spy, or turn an army of dretches into hezrous when marching against a rival lord. Demon Lords only rarely elevate demons to the highest ranks, fearful of inadvertently creating rivals to their own power.
Most other creatures believe demons to be little more than mindless engines of destruction and violence. If that supposition were true, the Blood War should have ended long ago, since the tactical and strategic genius of the Archdevils would have made quick work of the hordes that erupt from the Abyss. In fact, even though demons are quintessentially chaotic, their evilness combines with that outlook to produce a fiendish, subtle shrewdness.

Planar Invaders

Wherever they wander across the Abyss, demons search for portals to the other planes. They crave the chance to slip free of their native realm and spread their dark influence across the multiverse, undoing the works of the gods, tearing down civilizations, and reducing the cosmos to despair and ruin . . Some of the darkest legends of the mortal realm are built around the destruction wrought by demons set loose in the world. As such, even nations embroiled in bitter conflict will set their differences aside to help contain an outbreak of demons, or to seal off abyssal breaches before these fiends can break free.
Demons carry the stain of abyssal corruption with them, and their mere presence changes the world for the worse. Plants wither and die in areas where abyssal breaches and demons appear. Animals shun the sites where a demon has made a kill. The site of a demonic infestation might be fouled by a stench that never abates, by areas of bitter cold or burning heat, or by permanent shadows that mark the places where these fiends lingered.

Demonic Corruption

Despite the dark risks involved in dealing with fiends, the mortal realm is filled with creatures that covet demonic power. Demon Lords manipulate these mortal servants into performing ever greater acts of depravity, furthering the demon lord's ambitions in exchange for magic and other boons. However, a demon regards any mortals in its service as tools to use and then discard at its whim, consigning their mortal souls to the Abyss.
Few acts are as dangerous as summoning a demon, and even mages who bargain freely with devils fear the fiends of the Abyss. Though demons ,yearn to sow chaos on the Material Plane, they show no gratitude when brought there, raging against their prisons and demanding release
Those who would risk summoning a demon might do so to wrest information from it, press it into service, or send it on a mission that only a creature of absolute evil can complete. Preparation is key, and experienced summoners know the specific spells and magic items that can force a demon to bend to another's will. If a single mistake is made, a demon that breaks free shows no mercy as it makes its summoner the first victim of its wrath.

Binding a demon

The Book of Vile Darkness, the Black Scrolls of Ahm, and the Demonomicon of Jggwilv are the foremost authorities on demonic matters. These ancient tomes describe techniques that can trap the essence of a demon on the Material Plane, placing it within a weapon, idol, or piece of jewellery and preventing the fiend's return to the Abyss.
An object that binds a demon must be specially prepared with unholy incantations and innocent blood. It radiates a palpable evil, chilling and fouling the air around it. A creature that handles such an object experiences unsettling dreams and wicked impulses, but is able to control the demon whose essence is trapped within the object. Destroying the object frees the demon, which immediately seeks revenge against its binder.

Life beyond death

A powerful demon can take steps to safeguard its life essence, using secret methods and abyssal metals to create an amulet into which part of that essence is ceded. If the demon's abyssal form is ever destroyed, the amulet allows the fiend to reform at a time and place of its choosing.
Obtaining a demonic amulet is a dangerous enterprise, and simply seeking such a device risks drawing the attention of the demon that created it. A creature possessing a demonic amulet can exact favors from the demon whose life essence the amulet holdsor inflict great pain if the fiend resists. If an amulet is destroyed, the demon that created it is trapped in the Abyss for a year and a day.