But that claim is actually a stretch, if it is then it's very loosely based on a true story.
The latest being the pope's exorcist which I just watched.
After some googling
"The film is inspired by Amorth's books rather than a direct adaptation"
"In short, then, the film is a fictional tale based on the memoirs of a real man"
The film is based on his book, which consisted of his first hand claims, nothing second or third hand evidence, nothing to cross reference. It's hardly convincing.
Quite a stretch to claim it's based on a true story..