The Demon Sword Master of Excalibur Academy

Leonis, a powerful Dark Lord, wakes up from magical stasis after a thousand years to find himself in a ten-year-old boy's body. He soon meets Riselia, a girl fighting against the Voids, creatures that nearly wiped out humanity. To understand his new surroundings and possibly uncover a connection between the Voids and his past, Leonis enrolls in Excalibur Academy, a school that trains students to combat these mysterious enemies.