This is a documentary of the inspiration behind the movie The Exorcist. It features an interview with the author of the book The Exorcist is based on, and even delves into the original exorcism that was based on.
However, this documentary features and builds up to an exorcism performed by Father Amorth, an Italian Catholic Priest who is one of the best at what he does. Father Amorth is elderly and by the end of the documentary has already succumbed to pneumonia.
The host is William Friedkin, the producer and director of The Exorcist, he explores exorcism from a religious stand point, but interviews several neurologists, psychiatrists, psychologists, and other members of the clergy on their thoughts about the phenomena.
I have not yet watched the original movie, but I have watched other exorcism movies that are based on this idea. It is interesting to explore the basis of the movies and the myriad beliefs surrounding it.