Kai, a black magician, was betrayed by his younger brother, the hero Ares, and killed alongside the demon king. Hero Ares was trying to obtain the heart of saint Colette, who had feelings for Kai.

However, Kai suddenly regained consciousness and found himself participating in the Class ceremony that had happened five years ago. Knowing that his deviant class Black Magician was soon to be revealed, which would cause him to be despised as incompetent and exiled from the aristocracy once more, Kai made a decision.

With the level 999 [Power of Darkness] that he inherited from the struggles of his previous life, he would defeat the hero Ares and take back his childhood friend, Saint Colette. He would change his ruinous fate.

Kai, who has acquired the hidden class [Dark Star] due to his regression, gains the ability to control demons and plans his revenge on the hero's party and his family.

Kai acquires skills one after another and becomes stronger, eventually coming to be known as the strongest demon king in history.

The revenge tale of a black magician who will change the future and regain his loved ones begins!