Demon

Demons are a "race" of spirits. They do not reproduce, and are spiritual in nature. Their origin is not altogether clear, but their behavior is plain. They are the "embodiment" of malevolence, and drive primarily towards the suffering of others.
Occurrence of demons in the physical universe is rare in recorded history, and indeed there are countless multitudes more demons than their sightings would tend to imply. The few demons that do appear have done so largely due to prodding by mortals, or a special interest in particular mortals/extraordinary hatred and malice. By and large, the majority of demons which are to fill the Ethereal Horde at the end of the cycle are seemingly uninterested in, or unable to, parade around in the galaxy at large.
Those that do, or can, however, are a force to be reckoned with. Unable to be physically harmed, and generally able to withstand an individual's Magic, the ways to see them off are limited. Especially gifted "mages" are able to best demons by themselves, however certain artifacts and weapons also suffice. Two examples would be the Sha'bo'leh, a great spear seemingly designed to exile demons, and a particular Soul Hammer, which was able to trap a demon within it.