The film is based on the 2009 book The Making of a Modern Exorcist and actually somewhat closely parallels the true-life events chronicled in the tome.
As crazy as it sounds, The Blob was inspired by actual events.
The film purports to be based on actual events, but the slaughter of a group of hippies in rural Texas, as it’s depicted in the film, never really happened.
When it comes to horror films inspired by actual events, most fans know that The Conjuring is based on the case files of Ed and Lorraine Warren and that Henry: Portrait of a Serial Killer was loosely inspired by the confessions of Henry Lee Lucas.