18+ Gyakushuu no Dragon Rider

On the floating island of Lithos, war is ever-raging and there seems to be no end in sight as the three nations that live atop it vie for control. However, as the battles rage on, it is discovered that the crystal that makes the island's floating possible is diminishing in power and the risk of the island falling becoming ever closer to reality. Seeking to find another place to live, the three kingdoms appoint their own elite soldier, a "Dragon Rider," to fly to the ground below in search for the utopia known as "Arcadia" where there is no such thing as war.

(Source: MU)