The 1973 movie The Exorcist always makes the lists of the most horrifying movies. The 1971 novel the movie is based on is scarier, but only if you read the book before seeing the movie, in my experience. But neither is as terrifying as the original inspiration for the story, an actual exorcism that was performed on a 13-year-old boy in 1949.
Ronald Hunkeler heard strange sounds at his home in Maryland. First it was a dripping sound, then a scratching under the floorboards. The family could find no cause. Ronnie began to show strange symptoms, like scratches on his skin, and the family witnessed his bed shaking. He was moved to a relative's home in St. Louis, where things got worse. The scratches in his skin began to spell out words. Ronnie's behavior became more inexplicable, so Father Raymond J. Bishop was called in, and he brought in Father William S. Bowdern for an exorcism. Bishop kept a diary of what turned out to be a month-long exorcism, in terrifying detail. However, William Peter Blatty, who wrote The Exorcist, did not have access to the actual diary, although he did interview other involved. Read the story of the real-world exorcist at Dread Central. -via Strange Company