Shuuen no Elysion

Aoi and Kazuha, after a prolonged battle, mortally wound themselves while sealing the chaos king, a malevolent entity residing within human hearts. As they near death, one of Aoi's retainers betrays them, binding their souls to be reborn together to continue their struggle against the chaos king in every life. Centuries later, both are reborn as boys, and the chaos king is on the verge of awakening.