Mephisto is an extra-dimensional demon whose origin is unknown. He rules a fiery pocket dimension that he calls "Hell" or "Hades"
Mephisto's domain is inhabited by himself, by lesser demons who serve him, and by the astral forms of certain deceased human beings, which have been trapped in the bodies of demons. These demonic bodies imprisoning human spirits have been magically altered to resemble the deceased's mortal human forms.