Kyo Kara Maoh!

Kyo Kara Maoh! (今日から㋮王!, Kyō Kara Maō!, lit. Demon King from Today!) is a series of Japanese light novels written by Tomo Takabayashi and illustrated by Temari Matsumoto. The story follows the adventures of Yuri Shibuya, an average 15-year-old Japanese high school student, who is suddenly transported to another world where he is told that he is now the king of demons.
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.
The series was adapted into an anime in 2004 by NHK and a manga in 2005 serialized in Asuka magazine. The manga was updated for release by VIZ Media and launched in North America on September 30, 2014.