Leon Klein was a hero who saved the world by defeating the Demon King. However, due to the kingdom's betrayal, he was sent into a prison in the abyss. "Secret Quest".
In a hell with no exit, a trial that will not end until one million magical beasts are defeated. After 10 years, Leon acquired the [Dark Attribute Magic] and escaped the abyss.
But then, where he ends up in is another world. An academy that governs with magic. Moreover, he is forced to live as an unknown boy named "Kuroba Ren". This is a prestigious school that trains "Seekers". It is a place where people who defeats magical beasts and explore the unknown gather. And then, the one who rules this academy is... Leon's nemesis, who has once humiliated him before, Shark Crandall. "You, the one who took away everything from me is here, huh?" Though Leon borrows the body of an oppressed boy, the school's hierarchy and those who look down on him as "useless" was nothing to him." I'll reach the top, in my own way."