Demon

The generally humanoid demons' are not true demons, but are, in fact, 'tainted'. True Demons are huge monstrous creatures. These monstrous Demons are vastly powerful, but far from indestructible.

Demon bodies are composed of the same unnatural, necromantic energy as vampire bodies.

The lesser demons for the vast most part will tend to be anti-social, aggressive, violent, easily incited to anger and hatred, and generally just kind of mean, nasty folks. While their essential identities would survive the deaths of their bodies, without a 'soul' they have no direct connection to a Higher Power, and as such, their discarnate personalities would either wander around causing random amounts of mischief and harm out of malice and spite, get eaten by psychic predators, get enslaved to provide psychic power to necromancers, or wind up leaving the material dimension of Earth entirely and winding up in some demonic Hell realm where, they'd be tormented and, at best, end up in the service of some more powerful Demon lord there. All of which lead lesser demons with any intelligence and foresight to try to exercise one or both of two available options: either (a) making their physical bodies as close to immortal as possible to avoid the whole afterlife of torment thingie, or (b) making some kind of deal with some sort of greater Demon to enter into their service voluntarily, on this mortal plane and/or in the afterlife, so they would at least know where they're going to end up and what's going to happen there.

The gradual evolution of the soulless prehistorics into many different demon races, and the soulful primates into considerably fewer human races, is attributable to direct genetic and magical manipulation of the demonic races by Greater Demons, as opposed to a relative lack of such manipulation of human genetics by the Higher Powers.

There are lesser demon races here on Earth and Greater Demons dwelling in the Outer Dimensions trying to get back to Earth and all sort of demons of varying power levels and appearances and abilities in between living in other dimensions but traveling to Earth fairly often when summoned. Because they have at least some necromantic energy in their bodily make up, most lesser demons have adapted, over the generations, and developed the power to shapeshift into a semblance of human shape. Most lesser demons are stronger and tougher than normal humans. Some of the more powerful lesser demons can enter into pacts with Greater Demons for many different things, usually expressed as increased mortal power.

Despite the obvious fact that there are many physical differences between demons and humans (and between demons and demons, for that matter), the primary difference between the two groups is psychic, and defined by the presence or absence of a soul. This is underscored by the fact that demons can be nourished by human flesh, can use human bodies as incubators for their own young, and, especially, can interbreed with humanity in many cases and have fertile offspring.