The Exorcist: Legion - William Peter Blatty

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"