A Different World Dungeon Created With A ServantNewA Different World Dungeon Created With a Servant; Juuma to Tsukuru Isekai Dungeon; 従魔とつくる異世界ダンジョン; Make Dungeons in Another World with Familiars.

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: MAL > Square Enix, translated)