Demoniac

A demoniac is a person who becomes possessed by a demon. A demoniac undergoes a marked change in physical, mental, and emotional symptoms and behavior.

Description
Excerpts from  the Encyclopedia of Demons and Demonology:

Depending on the type of possession, there may be a pattern to the changes.

In ancient times, demons were blamed for entering aperson and taking control of him or her to cause problems. The ancient Jewish historian Josephus said that it was not a demon but the soul of a tormented person that entered a victim.

Demoniacs usually are not steadily possessed but act normally and then are overcome for periods of time. They may be possessed by multiple demons and have to undergo repeated exorcisms over long periods.