The Unreveal of whether Pazuzu was impersonating Damien's mother, or channeling her from Hell.
Fr. Merrin, the exorcism expert, believes the appearances of Mary and Burke and the "three personalities" Karras thinks are present are just more of Pazuzu's deceptions and "there is only one" demon.
Besides, Mary was very devout in her faith and proud of her son being a priest. She would be in heaven.
The movie doesn't even need anything supernatural to be terrifying; witness the scene of Regan getting a lumbar puncture.
Oh God. This troper could handle the demon's scenes easily, but it was the hospital scenes that made her cover her eyes in terror. Even more so since Regan doesn't seem to understand just why they're running so many tests on her and is frightened by it to the point of screaming and fighting the doctors.
In the book, Pazuzu mocks Karras for injecting Regan with so much Librium. He tells him (correctly) that he's going to induce a heart attack in the girl if he continues to.