Neo Arcadia - Yami ni Saku Niji

Once upon a time, the land was engulfed in complete darkness. The King of Light, who lived in heaven, wanted to create a nation of light on the land. He sent out his royal son, Rain of the Rainbow, to the King of Darkness, Zebb. The two instantly fell in love and did the opposite of what the King of Light wanted: they began living happily in the land of darkness. Suddenly, Rain goes missing one day...?!