Realms of Mythrais
Shadow Demons are the manifestation of a Fiend that is unable to fully project its presence into the world; very nearly like the Avatar of a Deity.
Shadow Demons have varied and horrifying powers, but they lack a true physical body. For this reason, possession is one of their favored tactics in the mortal world. By taking the body of a mortal, they are then able to more directly influence the world in a physical sense; and they lose none of their magical power by doing so.
Shadow Demons are difficult to defeat once they have possessed a mortal; even the destruction of the mortal’s body will not force a Shadow Demon from the world of Aerk. Instead, if it so chooses, it may very well attempt possession immediately of another host nearby; or instead it will simply appear to vanish so that it may find another host and then continue in its evil designs.
The principal purpose of a Shadow Demon manifesting is usually to possess a mortal and then start a cult dedicated toward the goal of summoning the full might of the Demon so that it can fulfill some twisted goal. Once a Shadow Demon has somehow caused its True Form to appear in the world, the Shadow Demon immediately merges into the full majesty of its True Form.