Possession is the hostile takeover of a living being by some kind of spiritual entity. Many religions recognize the existence of "demons" or "evil spirits" that can invade a human body and cause the victim to commit evil acts. Such beliefs have resulted in a variety of exorcism rituals intended to force such invaders to leave their victims. Such rituals tend to be, at the least, uncomfortable for the poor person forced to endure them. Such rituals are also quite useless, as the existence of possessing spirits has never been demonstrated in any scientific sense, and the symptoms of possession are generally the symptoms of mental or physical illness.
Possession in Fiction
As you might expect, possession is a rather common occurrence in fantasy stories.