William Peter Blatty
Before the movie, there was this book, a horror classic. Eleven-year-old Regan is possessed by a powerful demon. The first hint of trouble comes after Regan has been playing all day with a Ouija board and starts talking to Captain Howdy. Her mother dismisses it as a fantasy playmate. Soon even Mom smells fire and brimstone and its seriously downhill from there. Believe it or not, Blatty started his career writing comedies (A Shot in the Dark and Which Way to Mecca, Jack).