Kyo Kara Maoh! - God (?) Save Our King!

Historia

Yuri Shibuya was living a pretty normal life. That changed the day he was dunked into a toilet after an attempt to save a classmate from a gang of bullies. Instead of just getting a good soaking, he's pulled in. The next thing he knows, he's in a world that vaguely resembles medieval Europe. If that's not odd enough, he's told that he is to be the next Maoh, just because he has black hair and black eyes. The Maoh is the King of the Mazoku, who are coexisting not-so-peacefully with the humans in this world.

Sinopsis vía AniList (en inglés).