Daemonblight

Daemonblight (穢れ, Kegare; literally meaning "Karma or Resentful Demon Disease"), is the name assigned to the "disease" which appears and is used in both Tales of Berseria. It is a conditional disease which occurs through the events in the world of Wasteland and is the pre-context form of malevolence. Those who are corrupted or extensively exposed to a form of the daemonblight results in humans transforming into monstrous beings called Daemons. It began spreading rapidly after the Opening.

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 * —About Daemonblight.

Description
In actuality, "daemonblight" is the veiled term invented by the Abbey to refer to the corruption of humans due to malevolence, resulting in daemonic transformation. Malevolence has always existed and turned humans into daemons; however, it was the Opening that allowed for humans to see these daemons, fueling the myth of daemonblight and its sudden appearance in the world. This information is not common knowledge, however, as only high-rank exorcists and malakhim know of this fact; for the latter, the subject is taboo and not meant to spoken of. Eizen comes forward with this revelation when the group returns from Haria, having witnessed an outbreak of daemonblight in the village upon seizing the therion held within the Palamides Temple. In addition, it is later revealed that therions are daemon-eating daemons who actually consume malevolence, keeping the balance stable through seven earthpulse points scattered across the world.

Past
The Daemonblight had been bought into the Desolation world 10 years prior to the main story of Tales of Berseria, where the people opened the gate of hell unnoticed to them, which allowed the corrupting illness called the Daemonblight to spread in many sectors of the world.