18+ Ore dake Haireru Kakushi Dungeon: Kossori Kitaete Sekai Saikyou

Hidden dungeons—legendary places that no one knows how to enter—are full of extremely rare items, incredibly dangerous beasts, and strange beings that can grant power beyond measure. Noir, the third son of a penniless baronet, just lost his job. He was contemplating becoming an adventurer when fortune struck only a mile from home, and he opened the entrance to just such a dungeon!

(Source: Seven Seas Entertainment)