When a character's body is being controlled by other entity that took over their mind, either a ghost, parasite, eldritch abomination etc..
This is different from mind control in that possession featuring another person or entity is inhabiting their body and using it as their own, while mind control there they are being forced to do something by an external force that has taken over their mind (hypnosis or similar). This is usually indicated by a different reflection in a mirror, their eye color changing, or some other part of their appearance.
