Sister Ann has dealt with a demonic entity since childhood; it was possessed by her mother, causing her to exhibit disruptive behavior and violent acts, which eventually led to her death. When it suddenly returns in the form of Natalie, this becomes a major reason Ann joins the class taught by Father Quinn (Colin Salmon) on exorcisms and breaking gender norms within the church. However, it is never made clear exactly who or what the evil entity is.
The film’s title might allude to the being being simply “The Devil,” but there’s never really any confirmation. Honestly, Ann could just be dealing with a demon trying to cause some terror and pain in her life — and boy does it most certainly do. The demon’s disgusting, grotesque actions seem to indicate a severity of this presence, if not imply that it may be the main demon itself.