Source: varies (many)
Demons are chaotic evil outsiders native to the Abyss on the Outer Sphere of the Great Beyond. They are ferocious and irredeemably evil creatures. They attack and kill anything for the sheer fun of it, including other demons.
Demons are some of the oldest beings in the multiverse. The only beings of similar age are the elementals that inhabit the elemental planes which make up the Inner Sphere and the proteans who inhabit the Maelstrom.
The most powerful of the demons are unique creature with their own individual powers and abilities. These demon lords are known throughout the multiverse, and many cults exist to them on the Material Plane. The total number of demon lords is beyond knowing. Of the countless powerful demons who rule the various layers of the Abyss, some are better and more widely known and worshiped. The most ubiquitous demon lord, and the only one to have attained full godhood is Lamashtu, the Mother of Monsters. While Lamashtu's faithful can be found in nearly all parts of Golarion and beyond, the lesser demon lords receive their fair share of attention from their cults among the drow, the northern regions of Avistan (particularly the Worldwound), and the Mwangi Expanse.