A boy, Ulysses Van Flame, who has regained his memories of his previous life accidentally realizes that he is the last boss in an RPG game called “Abyss Fantasy”. With his knowledge of the original story, he avoids the death of his fiancée, the dark fall event, and while helping his subordinates to avoid falling into darkness, his goal is not To become a good guy, but to become a demon king, just like in the original story. Because with the knowledge of the original story, even the protagonist, who is a hero, and the world can be defeated. This is a tale of the dark deeds of a last boss who thinks that way and does not reflect on his actions at all.