The undead demon king Leonis sealed his soul for the decisive battle that will come in the future. However, when he woke up again 1,000 years later, he somehow turned into a 10-year-old boy! ? Protected by the beautiful girl Lyselia who belongs to the "Holy Sword Academy", Leonis enrolled in the "Holy Sword Academy" while being confused about this completely different world. The unknown enemy "Void", "Tactical City No. 07", and the "Holy Sword" that looks like a weapon but has supernatural powers. In this future world where magic has been forgotten, the school fantasy story of the holy sword and the devil sword intertwined by the strongest demon king and beautiful girls opens grandly!