Kono Ai Wa, Itan.

The following content is intended for mature audiences and may contain sexual themes, gore, violence and/or strong language. Discretion is advised.

Yoshino, a girl who spends her days in solitude stumbles across a mysterious book in a secondhand book store. She accidentally summons Belial the demon and he tells her she may trade one thing for a wish. Yoshino's wish? To be together with Belial until the day she dies... (source: aoc.moe)