Exorcism
In Catholic practice the person performing the exorcism, known as an exorcist, is an ordained priest. The exorcist recites prayers according to the rubrics of the rite, and may make use of religious materials such as icons and sacramentals. The exorcist invokes God—specifically the Name of Jesus—as well as members of the Church Triumphant and the Archangel Michael to intervene with the exorcism. According to Catholic understanding, several weekly exorcisms over many years are sometimes required to expel a deeply entrenched demon.
The Exorcist (film)
Lankester Merrin, a veteran Catholic priest who performed an exorcism in the 1950s, is on an archaeological dig in the ancient city of Hatra in Iraq. There he finds an amulet that resembles a statue of Pazuzu, a demon of ancient origins with whose history Merrin is familiar.
In Georgetown, actress Chris MacNeil is living on location with her 12-year-old daughter Regan; she is starring in a film about student activism directed by her friend and associate Burke Dennings. After playing with a Ouija board and contacting a supposedly imaginary friend whom she calls Captain Howdy, Regan begins acting strangely, including making mysterious noises, stealing, constantly using obscene language and exhibiting abnormal strength. Chris hosts a party, during which Regan comes downstairs unannounced, tells one of the guests—an astronaut—that he will die in space and then urinates on the floor. Later, Regan's bed begins to shake violently, adding further to her mother's horror. Chris consults a number of physicians, but Dr. Klein and his associates find nothing physiologically wrong with her daughter, despite Regan undergoing a battery of diagnostic tests. 
One night when Chris is out, Burke Dennings is babysitting a heavily sedated Regan. Chris returns to hear he has died falling out the window. Although this is assumed to have been an accident given Burke's history of heavy drinking, his death is investigated by Lieutenant William Kinderman. Kinderman interviews Chris. He also consults psychiatrist Father Damien Karras, recently shaken after the death of his frail mother.