[It's a familiar dungeon.]

Inside a medium-boss room of an RPG game that he played again and again until he can get there with his eyes closed. Waiting for a hero, a demon who exists only to be defeated, got his former life memory revived. Knowing that, in his previous life, he was a nothing-special college student... Continuing to stay as a to-be-defeated mid-boss is a no. The determined him, decided to defeat the boss and escape the dungeon.