SUGAR DARK: UMERARETA YAMI TO SHOUJO

The story follows a boy named Muoru who has been falsely arrested and sent to a cemetery to perform forced labor. There, he calls himself the "grave keeper" and meets a beautiful girl named Meria. Muoru becomes fascinated with Meria as he spends his days digging a hole containing the undead monster named "The Dark." (Source: Baka-Tsuki) — Description from AniList community (anilist.co).