Seiken Gakuin no Maken Tsukai

An undead sorcerer named Leonis Death Magnus, once the most powerful of the Dark Lords, had entered magical stasis during a decisive battle a thousand years ago. Upon awakening, he discovers a silver-haired young woman named Riselia in front of him. To his surprise, he has been changed into a ten-year-old child. He must now adjust to a new era while confronting dangerous creatures known as Voids. At the same time, he must conceal his true abilities from Riselia, who has taken on the role of his guardian.