"Demon" refers to the family of Celestial races which originate from The Abyss. They seek violent anarchy, wishing to destroy everything other than themselves. They grow stronger by slaying other creatures, which allows them to consume slivers of that creature's soul.

Relations of other Races
Demons are fearsome creatures, and avoided with extreme prejudice, lest one be slain by one and lose a piece of themselves forever.

Soul Feasters
When demons kill, they snatch up a piece of their victim's soul, using it to feed and empower themselves. Some demons, like the Barghest, are exceptionally efficient soul feasters, able to devour an entire soul if given enough time.