There are plenty of horror films out there that are based on true events. Usually these films are based very loosely on reality and leave you a little disappointed if you actually do the research.
There are terrifying supernatural horrors that are revealed to be massively exaggerated, or that have questionable sources. Slasher films based on notorious serial killers that have been given a generous dose of the Hollywood treatment. Details that don't serve the purpose of the story may be removed, characters or events may be added, and the end result no longer resembles the truth at all.
But what about when it happens the other way around and fiction becomes or even inspires reality? Horror movie sets that become victims of strange, ominous occurrences. Copycat killers that decide to act out their darkest movie fantasies. Even those who believe their favourite fictional villains have communicated to them directly, ordering them to carry out evil acts of their own.
They say that truth is stranger than fiction. And although it may be that in fact that's not always the case, what does stand out in the following entries is that when reality and fiction cross over, truth is inherently more terrifying than fiction.