A dark lord awakens from a thousand-year slumber, only to find the world under attack from monsters and so he must ally himself the humans he once fought against to stop them.

Awakening from magical stasis after a thousand years, the Dark Lord Leonis suddenly finds himself in the body of a ten-year-old boy. He quickly meets Riselia, a girl confronting the Voids, creatures that have nearly exterminated humanity. Determined to uncover the mysteries of this strange new era, Leonis enrolls in Excalibur Academy, a school that trains students to fight back against these enigmatic monsters. Could the Voids hold some connection to Leonis's past?

An undead sorcerer and powerful Dark Lord awakens as a ten-year-old boy a thousand years after entering magical stasis during a climactic battle. Now he must figure out how to adapt to a strange new era and deal with the twisted monsters called Voids.—HIDIVE

According to this tale, the demon king Leonis imprisoned himself for a thousand years to get ready for the decisive fight, but when he emerged, he discovered that he was now inhabiting the body of a 10-year-old kid. Then, a stunning student from the Holy Sword Academy by the name of Riselia asks him, "Why were you imprisoned in such a place? Now everything is OK; your big sister will take care of you." Leonis, who is under Riselia's custody, is dismayed to discover how drastically things have altered. Leonis finds himself enrolling in the Holy Sword Academy, bewildered and puzzled. What gonna Leonis do?—Bilibili TV

In preparation for an impending, decisive battle, the strongest Demon King, Leonis, sealed himself away. However, when he awakens 1000 years later, he finds himself reverted to the form of a 10-year old boy. "Why!?" "Why were you trapped inside of that? Don't worry. I'll protect you." Leonis is rescued by the beautiful girl Riselia, a student at Excalibur Academy, and is shocked to see how drastically the world has changed. The unknown enemy "Void", the "Strategic City No. 07" and the weapon-shaped force, the "Demon Sword" are all new terms to him. Despite being perplexed by these unfamiliar words, Leonis decides to enroll in Excalibur Academy. In a future world where magic has been lost, the curtain rises on a school sword-fantasy woven by the strongest Demon King and beautiful girls, involving both holy and demonic swords.—iQ