Seiken Gakuin no Makentsukai

A thousand years ago, Leonis Death Magnus was an undead king and one of the last Dark Lords. Realizing defeat was near, he sealed himself away with magic and planned to return in an age when he could rebuild his army. He awakens to two surprises: his body is now that of a ten-year-old human boy, and the world has forgotten the ancient wars and sorcery he knew. Humanity instead fights monsters called Voids with special powers manifested as Holy Swords. Riselia discovers Leonis and assumes he is an amnesiac child, bringing him to Excalibur Academy. Leonis still intends to reclaim his power and eventually conquer the world, but first he must understand this era and determine whether the Voids are connected to his forgotten past.