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.

Summary
Demon Possession's true title is the "Curse of Diablos" (ディアボロスの呪い), which only manifests in females because the heroes who fought Diablos were females. The Demon Possession that afflicts the women manifests itself as a form of magical overload that leads to the afflicted suffering from physical degeneration as it progresses, thus leaving the afflicted to become a lump of flesh until they die.

It manifests in the blood of the heroes' descendants to varying degrees depending on the lifespan of the race. At a certain stage depending upon the species' lifespan, the cells will begin to take over a female descendant's body and eventually transform them into a monster. 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 "Compatible" (適合者). 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.

Background
After being killed by the legendary Three Heroes, Diablos placed a curse on them so that they and their descendants would suffer painful deaths. The world has been led to believe the Cult's propaganda, as the reality it's that the Three Heroes were each implanted with cells taken from Diablos by the Cult.

Due to the sufferers being a descendant of the Three Heroes, The Church orders these individuals to be executed to free them from suffering by stating that those who are possessed by demons are monsters that can infect others who are close to them. They claim that they are protecting the common folk, but not before trying to use them for their experiments.

Curing Demon Possession
While Demon Possession can be cured with magic, it is said that the Cult of Diablos had tried in the past but they 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% just once.

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.

Eta's Theory Research on the Vampire Progenitor's blood
Eta was conducting an research about the Demon Possession on the Vampire Progenitor's blood, to which she acquire a certain hypothesis theory. Over the course of her research, Eta discover that there’s a possibility that the blood of the possessed and the blood of the Progenitors share the same origin and that the two merely diverged as they were inherited from parent to child.

For certain result, Beta will go to the Crimson Tower at the Lawless City, where the Vampire Progenitors reside and take a blood sample from a Progenitor, so Eta could check for her lab progress. Also, her job is to continue surveying the materials in the library and collecting anything which is important to further the theory research.

After Beta already securing the Blood Queen’s blood and seals it in a vial, she take them to the lab where Eta would analyze it. By the time of the conclusion research progress, Eta’s theory was proved to be valid. With the Vampiric power that could be useful together with proper training, Shadow Garden are able to mastering the Progenitors technique.

Trivia

 * Cid Kagenō figured out that Demon Possession is symptoms that afflicts the individual with high mana are at risk of losing control of it, which becomes a mutation state.
 * It was then Cid are able to know how to reverse the transformation by experimenting on the mutated Alpha, which not only restored Alpha but also granted her control over her increased mana.
 * During the experiments, Cid also figured out how to replicate the effects in himself, increasing his own power in the process.
 * Every member of Shadow Garden is a girl who succumbed to the "curse", which was subsequently healed and all of them from the descendants of the Three Heroes have at least decent magical abilities.
 * 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.
 * Although Alpha and Epsilon are the only ones so far that can heal the Curse of Diablos, apart from their lord Cid, apparently only their lord, can heal all kinds of afflictions and curses thrown from their enemies, like when he heal everyone which afflicted the Vampire's curse during the battle that Cid/Shadow had against Elizabeth at the red tower in the Lawless City.