Sister Ann has been dealing with a demonic entity since she was a young girl; it possessed her mother, causing her to exhibit some disturbing behavior and violent actions, eventually leading to her death. So, when it suddenly returns in the form of Natalie, it becomes a key reason Ann joins the class taught by Father Quinn (Colin Salmon) on exorcisms, breaking gender norms within the church. However, it's never made exactly clear who or what the evil entity is.
The film's title could allude to the entity being simply "The Devil," but there's never really any sort of confirmation. Truthfully, Ann could simply be dealing with a demon looking to cause some terror and pain in her life — and boy, it sure does. The demon's disgusting, grotesque acts seem to indicate a severity to this presence, if not perhaps implying it might be the head demon himself.
