Based on a true story

'The Exorcist' tells the terrifying tale of young girl Regen MacNeil who becomes possessed by a demon, but did you know that the story is based on a real life possession?

In 1971, writer William Peter Blatty published his novel of the same name, which is based on the apparent possession of a 14-year-old boy from Maryland in 1949. The boy gained incredible strength, could distort his body in unexplainable ways and began to talk as if a sinister entity had taken control of him.

After doctors and psychiatrists tried and failed to relieve the boy, his parents were shocked to discover red marks spelling out the words 'Go' and 'St. Louis' appear on his back, and so the family moved to the Missouri city. There, they met the Jesuit priests Father Walter Halloran and Reverend William S. Bowdern who performed a three-month exorcism to free the boy from the demon's grip.