Dungeon no Osananajimi is a Manga/Manhwa/Manhua in english language, MONSTERS series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

Dungeon x Childhood Friend!? Let's watch over them, in this warm romantic comedy! At the bottom of an impregnable underground dungeon awaits the final boss, the mighty black dragon Obsidian. The one who dared to challenge Obsidian is Van, an experienced adventurer who has fought many battles with the dragon over the years. The battle between two passionate souls is about to begin—!