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, ebb. The two instantly fall in love and do the opposite of what the King of Light wanted; they begin living happily in the Kingdom of Darkness. Suddenly, Rain goes missing one day...?!

(Source: Dangerous Pleasure)