Seiken Gakuin no Maken Tsukai

A thousand years ago, the six Dark Lords imposed their control over humanity, driving it to the brink of extinction. However, all was not lost. Six Heroes rose up against the Dark Lords and eliminated them one by one with their powers. However, before dying, the Undead King, Lord Leonis Death Magnus, sealed his own body within a powerful magical barrier and cast a resurrection spell upon himself. His objective was clear: to return in a thousand years to conquer humanity, in order to fulfill the mission entrusted to him by the Goddess of Rebellion. Several hundred years later, Riselia Ray Crystalia chanced upon ancient ruins that had belonged to Leonis. While exploring the site, the young woman accidentally destroyed the Dark Lord's seal, pulling him from the millennial slumber in which he was immersed...