Kassiyadin (Old Exekian, "spiritual warfare", "battle with the supernatural" or "conflict with the unseen") is a religious concept common in most Akhenist sects.
Adherents believe in a supernatural or spiritual realm beyond the physical plane of reality. This world includes the spirits of the unborn, angels, demons, and other such beings. Some of these beings are hostile to the living and will attempt to attack or sabotage them, and the living person may fight back. Thus kassiyadin, or spiritual warfare, occurs.
Some of the founders of Vormundism (including Rowan Renard and Gavin Zosimus) disputed some of the teachings regarding kassiyadin, attributing things like demonic possession to soul-sickness. They do not deny the existence of the supernatural but do not approve of the methods which the paladins used in dealing with it. The prophet Dagda Vormund himself apparently engaged in a kind of amateur kassiyadin. His attempt to help his mentally ill follower Ethan in this way proved unsuccessful, though Ethan's relapse into madness and violence can be attributed to his having been drugged with the hallucinogenic yignaal.