Kyo Kara MAOH!

Born and raised in Japan for 15 years, Yuri Shibuya, an ordinary baseball boy, is suddenly transported to another world one day. He ends up in a world where people with black eyes and black hair are feared as "devil lords"... Surrounded by his beautiful and unique subjects, His Majesty Yuri has been appointed as the 27th Demon King of the Great Demon Kingdom. What will tomorrow be like? Delivered full of original elements!

[Web manga on Comic Bushiroad Web](https://comic-growl.com/episode/13933686331795933166)

Kyou Kara Ma no Tsuku Jiyuugyou is the novel the anime and manga are based on. It's about a boy named Yuuri, who gets himself flushed down a toilet into another world. There, he finds out he's the Maoh (demon king) and that the Mazoku (demons) expect him to lead them against the human race.
The novel is told from Yuuri's PoV, with plenty of side-commentary that is left out of the show.