Legion by William Peter Blatty
Legion is a 1983 horror novel by American writer William Peter Blatty. A sequel to The Exorcist, it also involves demonic possession and was adapted for the film The Exorcist III in 1990. The book was the focus of a court case over its exclusion from The New York Times foremost Seller list. Blatty based aspects of the Gemini Killer on the real-life Zodiac Killer.