Story
When his total scumbags of parents die, he finds himself with a mysterious inheritance of 500 million yen. Deciding to do whatever he likes from then on, the boy enters the most challenging online game, “Dungeon Attack”, where he becomes the first to unlock the achievement of Defeating all the Demon Lords. However, when he answers a mysterious question that appears after completing the game, he suddenly finds himself sent into the game world. He had been Reincarnated as “Dantalian,” the Lowest and Weakest Demon Lord, Ranked 71st in the game world!
Synopsis via AniList (English).