[@Zombiedude101] Current idea is a ghost/revenant/wraith who consumes souls to grow stronger. More specifically, the more souls they consume the more they can interact with the physical world and the more their power grows. If "starved" they lose their power over time and revert to a state on par with a basic poltergeist and can't do much more than some weak telekinesis and phase through walls. They're currently contained in a simple locked room with all manner of occult symbolism on all surfaces. Thermal imaging is used to monitor them as they're very nearly invisible to the naked eye.