Story:

One of the most powerful demon lords ever has been sealed away for a thousand years. When he finally awakens, he is reincarnated as a 10-year- old boy. Now under the care of the girl who released him, the demon lord Leonis must navigate the perils of Excalibur Academy.

(story from Manga)

Story by Yen Press:

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?

External Link: Official Website