The Demon Sword Master of Excalibur Academy, Vol. 1 (manga)

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