MY BRIDE IS DEMON LORD GN

Humanity, once at war with a demonic army, ended when a hero felled the leader of the monster horde, The Demon Lord. Flash forward to a time of peace and the sorcery school of Seirou Academy, built on those ancient battlegrounds, is an institute where students learn ancient magic and enter friendly competitions between countries. The newest transfer student Lina, the clumsy grandchild of The Demon Lord, has her sights on Yuuya, the descendant of the legendary hero, and she's determined to make him her bride. Problem is that once word gets out that he's the Hero's decedent, a whole harem of girls are going to be interested in his 'sacred sword'!