The Sea of Japan (日本海, Nihonkai) is a large mass of water bordering Japan and the Continent. Beneath it's watery surface is a kingdom of sea yokai which is ruled by the daiyokai, Shokujin Tenchikaimei. There is not much else known about it at the moment.
Long ago, the Leviathan, a great yokai that was created around the beginning of the earth, arrived. It's yoki was so strong that few could live in it's presence. It dominated the oceans for many centuries until it reached the Sea of Japan. There, it did battle with the great yokai kingdom that resided in the sea. As a last resort, the king sealed his youngest son in sword form, connecting to a seal on the Leviathan itself. In order to maintain the seal, the process must be repeated once every century.