In some parts of Christianity, demons are said to be able to possess humans. In order to force the demon to leave the body of the victim, an exorcism is performed.
At places where belief in ghosts prevail, a ghost can be driven from a haunt (often a house, manor or castle) by an exorcism.
The exorcist is often a member of the local clergy, thought to be able to assail the spirits with divine help, or an magician of some sort.
For a discussion of the validity of possession, see Spiritual possession.
Film: The Exorcist (1973), and its sequels.
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