The Demon Sword Master of Excalibur Academy(2019)

Seiken Gakuin no Maken Tsukai

One thousand years ago, Leonis Death Magnus, undead sorcerer and the greatest of the Dark Lords, entered magical stasis during a climactic battle. When he awakens, he finds a beautiful silver-haired young woman named Riselia standing before him. More shockingly, Leonis has been transformed into a ten-year-old boy! Now he’ll have to adapt to this strange era and deal with the twisted monsters called Voids-all while keeping his true powers a secret from his lovely new guardian.

(Source: Yen Press)