The Exorcist was made into the movie "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 1973 American supernatural horror film directed by William Friedkin, adapted by William Peter Blatty from his 1971 novel of the
The Exorcist III film), as the protagonist. Blatty conceived "Legion" as a feature film with William Friedkin, director of "The Exorcist", attached to direct. Despite the critical and commercial failure of the previous sequel, Warner Bros. was keen to proceed with Blatty and Friedkin's plans for another "Exorcist" film. Blatty said that "everybody wanted "Exorcist III"... I hadn't written the script, but I had the story in my head...and Billy [Friedkin] loved it." Friedkin, however, soon left the project due to conflicting opinions between him and Blatty on the film. The project went into development hell, and Blatty wrote "Legion" as a novel