The higher levels even after vanquish.
These things, are the spirits of one of a kind high level demons.
They are balls of light that chase after mortal souls to steal the bodies as a host for their own spirits.
They have 24 hours to find a body, in that first 24 hours of possession the demon cannot be active if the body is unconscious exposing vulnerability.
They are unable to possess a body that contains even a single drop of magical blood. The saving of a possessed mortal requires a “blood oath” or “blood promise”.