Juuma to Tsukuru Isekai Dungeon(2021)

Juuma to Tsukuru Isekai Dungeon

One day, Nagayuki Karasumaru discovers that his closet contains a cavern linked to another world. In that cavern, he finds small creatures on the verge of death. Upon saving them, Nagayuki notices he has become a dungeon master.

(Source: Square Enix, translated)