Shuuen no Elysion

After a long fight and receiving a fatal wound, Aoi and his loyal retainer Kazuha sealed the chaos king, a demonic being that lurks in human's hearts... but in their last moments another of Aoi's retainers betrays him and chains Aoi and Kazuha's souls to reincarnate together in order to keep fighting the chaos king in all their lives. Centuries later, what will happen now both are reincarnated as boys and the chaos king is about to wake up?