1. The Amityville Horror
The Original: The Amityville Horror (1979)The Lost Sequels: All of themInvalidated By: A lawsuit Although it hasn't necessarily aged well, the original The Amityville Horror film from 1979 is often regarded as a horror classic. It was a massive blockbuster upon release, largely thanks to the worldwide infamy of the book and allegedly true story it was based on. It told the story of the Lutzes, a family of five who moved into a large house in Amityville, New York after getting a great deal on it because a young man named Ronald DeFeo Jr. had shot his entire family to death inside just a year earlier. They lived in the house for just 28 days before leaving it behind, along with all of their possessions, claiming to have experienced escalating ghostly and demonic phenomena. The Lutz family was initially reluctant to allow a movie to be made of their experiences, especially since the film rights to their story had been sold by the book's publisher without their consent. To make up for this somewhat, the Lutzes were given the sequel rights to the film - which, of course, proved to be very valuable when the movie became a smash hit. The Lutzes weren't interested in making a new film, though - instead, they continued to consult on a series of books that were increasingly loosely based on their experiences, beginning with The Amityville Horror II by John G. Jones in 1981. Because Amityville is a real place, as is the infamous haunted house and the definitely-true story of the murders, anyone can write an Amityville book or make an Amityville movie. The Lutzes retained copyright only of the stories based on their own claimed experiences and any sequels to their own story. Plenty of other books had been written on the subject by avoiding the Lutz story entirely, including Murder in Amityville by Hans Holzer. That book told the story of the DeFeo murders in a predictably exaggerated and supernaturally-tinged fashion, and was picked up for a film adaptation by producer Dino DeLaurentiis. Since the Lutzes held the copyright to the title The Amityville Horror, the DeLaurentiis film was titled Amityville II: The Possession. It went so far as to change the name of the DeFeo family to Montelli, fictionalize (and of course, sensationalize) the events heavily, and make absolutely no mention of the Lutzes or anything that allegedly happened to them. However, as the release approached, someone made the mistake of explicitly referencing the Lutzes on the posters. This led to a lawsuit over the rights to their story and any sequels, and ultimately Amityville II's distributor was forced to print a new poster that consisted entirely of a legal disclaimer stating their new movie was not a sequel to The Amityville Horror, was not based on th book The Amityville Horror II, and had nothing to do with the Lutz family (though perplexingly they were allowed to keep the II in the title).
The movie was not a major hit like the first film, but still managed to turn a profit. DeLaurentiis produced one additional film, Amityville 3D, which told an entirely original fictional story that made no references to the Lutzes OR Amityville II: The Possession. Though that was the last theatrical release in the series, the franchise continued on as a series of made-for-television and direct-to-video releases all the way through 1996's Amityville Dollhouse. Apparently out of an abundance of caution, not only are none of them sequels to the first film - none of them are sequels to any other film in the series either, and in fact, most contain explicit story contradictions with other entries. The series is based on a shared theme rather than any actual unifying narrative. It's also worth noting that the production and release of the 2005 remake of the original film saw a considerable legal challenge from George Lutz, who was not consulted on the project and claimed it was such an inaccurate depiction of events as to be defamatory (among other things, it shows him killing his family dog and attempting to kill his wife and kids - neither of which ever happened even in his most intense tellings of the tale). Sadly, George passed away before a satisfactory conclusion to the matter could be reached.