Demon Possession

Demon Possession (悪魔憑キ) is a disease and magic curse cast by the demon king, Diabolos, that affects people born with the demon blood of Diabolos in their veins due to being the descendants of Heroes that defeated him. It is a sudden event where the body begins to rot until it is not recognizable as a human being anymore.

The Church executes these individuals to free them from suffering, while at the same time stating that those with demon possession are monsters. They claim that they are protecting the common folk.

Summary
Demon Possession's true title is the Curse of Diabolos, which only manifests in females because the heroes who fought Diabolos were females. It manifests in the blood of the heroes' descendants to varying degrees depending on the lifespan of the race. The greatest percentage of those who manifest are elves, due to their long lifespan they are fewer generations removed from the time Diabolos was slain. The beastkins follow the elves, and finally humans.

The Order of Diabolos, with its roots deep in the Church, prioritizes hunting and executing those who have demon possession. They call these individuals "Matches."

Curing Demon Possession
Early in his years, Cid Kagenou discovers one of these people undergoing the effects of Demon Possession and trains his ability to use magic through the individual's enormous amount of magic power seeping from it. In consequence, Cid gained an enormous amount of magic power and control. When he completely suppressed the magic deviation, magic that was going out of control inside the body, Alpha was in front of him.

All of the Seven Shadows of Shadow Garden had Demon Possession until Cid cured them.

Cid also accidentally cured his sister from demonic possession when they were younger, thinking it an error in her mana flow.