Since a ghost is a soul with a mind, and aer is the mind detached from body and soul, a ghoul can attain a mind, should it come into contact with high densities of aer. Should this happen, the mind makes the ghoul conscious, but as it is not their original mind, they have effectively become a more dangerous, violent ghoul. These creatures are what Unitians call monsters.
Fiends are especially foul Monsters, Ghouls, and Ghosts from Nyrellan mythology who are said to be the “main antagonists” of the religion, existing to sow chaos, distress, and death across the world. Their strength is much greater than that of the typical monster.
These disturbed souls, whether they be ghosts, ghouls, monsters, or fiends can only be truly defeated if they are exorcized by priests of the Church. Really, anyone can exorcize them, so long as they know the correct procedures.
Just about anything holy can weaken a monster or a fiend and make them easier to deal with. The Nyrellan Hymn, for example, can weaken but not exorcize them.
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