Legion is a 1983 horror novel by American writer William Peter Blatty, a sequel to The Exorcist. It was adapted for the film The Exorcist III in 1990. Like The Exorcist, it involves demonic possession. The book was the focus of a court case over its exclusion from The New York Times Best Seller list. Blatty based aspects of the Gemini Killer on the real life Zodiac Killer, who in a January 1974 letter to the San Francisco Chronicle had praised the original Exorcist film as "the best satirical comedy that I have ever seen".

The Exorcist is a 1971 horror novel written by American writer William Peter Blatty and published by Harper & Row. The book details the demonic possession of eleven-year-old Regan MacNeil, the daughter of a famous actress, and the two priests who attempt to exorcise the demon. The novel was the basis of a highly successful film adaptation released two years later, whose screenplay was also written and produced by Blatty. More movies and books were eventually added to The Exorcist franchise.