Kyo Kara MAOH - Volume 3

When Japanese schoolboy Yuri Shibuya, who has a strong sense of justice, gets flushed into another world, he is hailed as the king of the Mazoku, beautiful demons who want him to lead them in their war against humans.

Yuri becomes the king of a nation where all of the citizens are demons, but they appear indistinguishable from humans. Their only distinguishing traits are their long lives and the ability to use magic. The people of the Demon Tribe are able to make a pact with an element after which they can then use magic of that element. Covenant Castle is in the capital of the Demon Kingdom. It is the residence of the demon king. The culture of The Great Demon Kingdom is very different from the Japanese culture Yuri is accustomed to, and the differences make for amusing mishaps with long ranging consequences, such as an accidental proposal of marriage.