Made a VLOG about why i love this movie if anyone is interested:

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I love that you are so passionate about this film. I, too, believe it to be a lost horror gem that will hopefully garner more attention as time goes by.

However, you are incorrect in your description of the plot. It is, in fact, Damian in the hospital, not someone who looks like him. And he is possessed not just by the demon who possessed Regan, but also the spirit of the serial killer James Vennamun. Once Vennamun was executed, the demon moved his spirt into Damian's body. It then took 15 years for them to "repair" Damian's broken body after falling down the stairs in the original Exorcist. That's why 15 years passed between Vennamun's killings. Remember, at one point, Vennamun tells Kinderman, "He is inside with us! His pain won't end!"

At the end, Kinderman shoots Damian, who is able to briefly fight the demon and Vennamun and become himself for a brief moment.

As you watch the movie again, this will all become clear. It is a movie that demands multiple viewings.