Seiken Gakuin no Maken Tsukai

A thousand years ago, Leonis Death Magnus, a powerful undead sorcerer and Dark Lord, went into magical stasis during a major battle. Upon waking, he discovers a silver-haired young woman named Riselia in front of him. He is also surprised to find himself transformed into a ten-year-old boy. He must now adjust to this new time period and face the dangerous creatures known as Voids, all while concealing his true abilities from his new guardian.