is a japanese light novel series. it was written by kugane maruyama and the picture in the series are drawn by so-bin. sixteen volumes of this light novel have been published since july 30, 2012. the author has said that he has planned for 18 novels. there is a manga based on this light novel by satoshi ōshio. the art in the manga is by hugin miyama. it started from november 26, 2014 in kadokawa shoten's manga magazine "comp ace".