A thousand years ago, the world was at war. Led by the Six Dark Lords, humanity as a whole was nearly led to the brink of extinction. However, humanity had hope in the form of the Six Heroes. With their power, the Six Dark Lords fell one by one until finally, only one remained. In the year 447 of the Holy Calendar, the Undead King, Leonis Death Magnus, also fell but before his capital, Necrozoa, was conquered, he sealed his own body with a powerful barrier and cast a resurrection spell on himself. He declared that he will concede victory only this once but he shall rise again in one thousand years to conquer humanity for that is the mission given to him by the Goddess of Rebellion.

In the present time, a young girl named Riselia Ray Crystalia stumbled upon the ancient ruins of Leonis's mausoleum. Accidentally undoing the seal, Leonis awakens after his thousand-year slumber in the body of a ten-year-old human child. Mistaken to be a refugee of the Voids, Riselia takes Leonis into her care and Leonis begins his mission to uncover what has happened to the world while he slept.