Spawn of Chaos. The Abyss creates demons as extensions of itself, spontaneously forming fiends out of filth and carnage. Some are unique monstrosities, while others represent uniform strains virtually identical to each other. Other demons (such as manes) are created from mortal souls shunned or cursed by the gods, or which are otherwise trapped in the Abyss.
Capricious Elevation. Demons respect power and power alone. A greater demon commands 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.