Horizon in the Middle of Nowhere

When humans came down from the sky they brought with them the Testament, the guide to the path they must follow if they wish to return to the skies again. Now, in a strange world where only the islands of Japan are inhabitable, the nations of the world vie for power and protect the portions of Japan that they have claimed. Each is armed with its own ultimate weapon: a Roysmoi Opro, an Armor of Deadly Sins....