|Fantasy game art from Dragon Age: Origins Ultimate Edition (Dragon Age: Origins, the Awakening expansion, and all official Downloadable Content).
Arcane Horrors are corpses of mages possessed by Pride Demons -- the strongest demons from the Fade in the world of Dragon Age Origins.
Demons, of course, have no form in our world. When they enter, either where the Veil is particularly thin or through blood magic summoning, they must take possession of a body. When a pride demon takes control of the corpse of a mage, an arcane horror is born. Although they appear to be little more than bones, these are fierce creatures, possessing not only all the spellcasting abilities of a living mage, but also the capacity to heal and even command other animated corpses.