 William Peter Blatty (qv) made the film based on his novel  "Legion" which was published on 1 August 1983. He was extremely  reluctant to sell the movie rights after the sequel but eventually  decided to step up to the directors chair himself. The company  decided to call it "The Exorcist III" even though the screenplay and  novel featured no exorcisms whatsoever. When the company realized  this as the film was going into shooting, they demanded an exorcism  scene be written and added into the film.