Exorcist

William Peter Blatty's The Exorcist (1971) follows two Catholic priests trying to save a twelve-year-old girl from a violent demonic possession, and became the basis of the acclaimed 1973 film. Blatty returned to the story in 1983 with Legion, in which police detective Bill Kinderman's investigation of a string of ritual murders leads him back to the aftermath of that exorcism and the fate of Father Karras; Legion was later filmed as The Exorcist III. Other Exorcist books are tie-in and franchise material rather than further novels by Blatty himself.