The demons are nefarious dark entities that attack the human soul, according to the teachings of the Chuch.
The Church claims that demons exist and can possess people to perform their nefarious deeds in this world. The sad thing is that this seems to be true. Even the most rational scientists must admit to the reality of something out there, something often unseen but seemingly hostile. These entities, demons or aliens or whatever, can so far only be combated by Church theurgy and miracles of faith.