the monster (abraham) possessing a priest (tom)
the last demon who was possessing father tom's body was killed but having already been possessed made him vulnerable to other demons and BOY did he get a bad one
abraham hardly qualifies as a demon, he's more like an almost infinitely powerful entity who deals in corruption on all levels
he has constant control over tom, who's conscious is forever locked away in abraham's personal limbo-like realm that once imprisoned him