This story is taken from my favorite Manga/Anime in my lifetime, Kyou Kara Maoh! (今日からマのつく自由業!) So I give the credit to the owner and this is not purely my own idea because yes, I do take the plot based on the comic with changes of course ( to match with the characters )
I hope to get a lot of people to read this! 

Lee Sungyeol 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 King, just because he has black hair and black eyes. The King of the Mazoku, who are coexisting not-so-peacefully with the humans in this world. Much to his subject's dismay, he's totally different from the rulers they're accustomed to. He's kind, considerate, a believer in justice, and not willing to use violence to solve conflicts. Not exactly someone they want running a country on the very brink of war. Now, Sungyeol has to deal with trying to become a good King, while at the same time attempting to adapt to this land's customs and culture, all in a world where the tension between the humans and King is reaching its peak.

http://www.asianfanfics.com/story/view/248510/god-save-our-king-comedy-hoya-infinite-myungsoo-sungjong-sungyeol-woohyun