Seiken Gakuin no Makentsukai S01

A thousand years ago, Leonis Death Magnus, the undead sorcerer and greatest Dark Lord, was put into a magical stasis during an exciting battle. When he awakens, he finds himself face to face with a beautiful silver-haired young woman named Riselia. More shocking is that Leonis has been transformed into a ten-year-old boy! Now he must adapt to this strange new era and get along with twisted creatures known as the Voids, all while keeping his true power hidden from his new beloved guardian.