Demon Possession

Demon Possession (悪魔憑き) is a disease and magic curse cast by the demon king, Diablos, that affects people born with the demon Diablos' blood in their veins due to being the descendants of the Three 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 Diablos", 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 Diablos was slain. The elves, follow the beastkins, and finally humans.

The Cult of Diablos, with its roots deep in the Church, prioritizes hunting and executing those who have demon possession. They call these individuals "Matches." The reason the Cult try to find any individual which already have a possessed symptoms is because they become a main ingredient to create Tears of Diablos.

Curing Demon Possession
While Demonic Possession can be cured with magic, it is said that the Cult of Diablos, tried in the past but couldn't, create a cure, as the curse was very powerful comparable to a Demon king, assuming it wasn't there was a cure for it. That is until Cid Kagenō in his young age manages to 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 Cid completely suppressed the magic deviation that was going out of control inside the possessed body, he accidentally managed to heal Alpha. A side effect is that the afflicted's magical power will increase a bit, after being healed of their curse, but only by 5-10%.

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.

Before or after Seven Shadows leave their lord, Cid, Alpha and Epsilon are able to cure the people who had the Demon Possession beside Cid and they are able to recover any individual who have a possessed symptoms, also subsequently recruited them into the Shadow Garden organization when the "Seven Shadows" began their membership expansion.

Trivia

 * During membership recruitment, Eta created a device known as Locator of Possess, that which could track anyone with possession symptoms who would later be cured and recruited by Shadow Garden.