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 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". Source: Wikipedia (en)