The Exorcist (1973)
Based on the 1971 novel by William Peter Blatty, The Exorcist marries three
different scenarios into one extraordinary plot. A visiting actress in
Washington, D.C., notices dramatic and dangerous changes in the behavior
and physical make-up of her 12-year-old daughter. Meanwhile, a young
priest at nearby Georgetown University begins to doubt his faith while dealing
with his mother's terminal sickness. And, book-ending the story, a frail,
elderly priest recognizes the necessity for a show-down with an old
demonic
enemy.

Exorcist II: The Heretic (1977)
The demon is back in this the second film of the three part series as a new
priest played by Richard Burton tries to find the demon still living inside
Regan (Linda Blair).