Demons are almost universally evil creatures that fall under one of two designations: natural creatures corrupted by evil spirits, or malign beings native to the realm of Heck. Demons can be found serving many evil powers, such as Stragath or Akriloth. Aside from powerful demons like Stragath, most demons seem to have no purpose other than the corruption or destruction of all life; as such, they are frequently harnassed by powerful villains as foot soldiers.
Not all demons are evil; Lady Tomo on Isle d'Oriens is a prominent example.