
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.
Maruma Series is a Japanese novel of 1,752,579 characters, with 34,940 unique words and 2,979 different kanji. Its difficulty rating on Jiten is hard, 3.5 out of 5. The full vocabulary list ranks all 34,940 words by how often they appear, and you can study them on Jiten or download them as an Anki deck.
Here are some of the words that appear most often in this media, as well as a sample of its rarer vocabulary.