Demon

The natural inhabitants of hell, demons were born from the corpse of the swirling cloud of chaos that first created life within the Barren. They tend to come in much more bestial or metaphysical forms than their unnatural brethren (Devils) but can still have incredible intelligence and wits. Monsters, plagues, info hazards, objects, all of which can be the form of a demon.
With these creatures having radically different forms, it's challenging to understand their motives. What is known is that the appearance of demons in the mortal realm is always a sign of disaster and death. Unlike Devils, wits and cunning won't be enough to defeat a demon, nor physical strength - to defeat one, one must first learn the history of the demon in question (or its family). Though this is assuming that you know you're facing a demon. If it comes in the form of a plague, civilization may focus on developments on cures or quarantine without knowing the true answer is within ancient books of demonic lore.

Related Myths

• The War Between Heaven and the Hells