From The Art and Popular Culture Encyclopedia
The Exorcist is a horror novel written by William Peter Blatty. It is based on a 1949 exorcism Blatty heard about while he was a student in the class of 1950 at Georgetown University, a Jesuit and Catholic school. The exorcism was partially performed in both Mt. Rainier, Maryland  and Bel-Nor, Missouri.  Several area newspapers reported on a speech a priest gave to an amateur parapsychology society, in which he claimed to have exorcised a demon from a thirteen-year-old boy named Robbie, and that the ordeal lasted a little more than six weeks, ending on April 19, 1949.
An elderly Jesuit priest named Lankester Merrin is leading an archaeological dig in northern Iraq and studying ancient relics. Following the discovery of a small statue of the demon Pazuzu (an actual ancient Sumerian demigod) and a modern-day St. Christopher medal curiously juxtaposed together at the site, a series of omens alerts him to a pending confrontation with a powerful evil, which unknown to the audience at this point, he has battled before in an exorcism in Africa. Meanwhile in Georgetown, a young girl named Regan MacNeil living with her famous actress mother, Chris MacNeil, becomes inexplicably ill. After a gradual series of poltergeist-like disturbances, she undergoes disturbing psychological and physical changes, appearing to become "possessed" by a demonic spirit.
After several unsuccessful psychiatric and medical treatments, Regan's mother turns to a local Jesuit priest. Father Damien Karras, who is currently going through a personal crisis of faith after the loss of his mother, agrees to see Regan as a psychiatrist, but initially resists the notion that it is an actual demonic possession. After a few meetings with the child, now completely inhabited by a diabolical personality, he turns to the local bishop for permission to perform an exorcism on the child. After consultation with the Jesuit president of Georgetown, the bishop appoints the experienced Father Merrin, recently returned to the States, to perform the exorcism and allows the doubt-ridden Karras to assist him. The demon supposedly possessing Regan calls himself Pazuzu, the same demon whose statue was found earlier.
The lengthy exorcism tests the priests, both physically and spiritually. After the death of Father Merrin, the task ultimately restores Father Karras' faith, leading him to give his own life to save Regan's.
The novel was made into a successful Academy Award-winning horror film in 1973, with the screenplay written by Blatty. The film originally contained several key sequences from the novel, which were cut prior to release by director Friedkin over Blatty's protests. The deleted scenes—and some new digital effects—were inserted into the re-release subtitled "The Version You've Never Seen" in 2000.