What every horror film loves to boast, if possible, is the claim of "Based on a True Story." Because if the story is not just from some writer's imagination, if there is a thread to an actual event with real people, no matter how thin, then the fear becomes more real. William Friedkin's 1973 motion picture The Exorcist is considered one of, if not the scariest horror film of all time. At that time, no audience had seen a case of demonic possession dramatized so deftly and explicitly, and the resulting shock left a lasting mark on popular culture and our collective psyche. Not as widely known is that William Peter Blatty adapted his Oscar-winning screenplay for the movie from his novel of the same name. And perhaps even lesser-known to most is that Blatty based his book on a real-life case, what is more commonly known as the "1949 Exorcism," or "The Exorcism of Roland Doe," or of "Robbie Mannheim."