18+ Magan de Hajimeru Gekokujou

A treasure with the "power to change the world" sleeps in a certain ruinous dungeon. Now there were those who aimed at that treasure and challenged the dungeon infested with ancient creatures. First, Princess Knight Lilithia, the first princess of the Kingdom of Langbird. Next, the soldiers of the Georgian Empire, who are hostile to the Kingdom of Langbird. And a young man, Eight, who has just become a slave of Lilithia for some reason. Eight was unintentionally embroiled in a battle to scramble for a treasure that could affect the survival of the nation, but when he approached the depths of the dungeon, he heard a girl's voice out of nowhere!?

(Source: Kadokawa, translated)