'As soon as he opens his eyes, the man realizes he has been reincarnated into the world of the famous RPG series 'Spirit Wars: Dungeon Magia.' However, it isn't as the protagonist that he has been reincarnated, nor even as a background character… With weak abilities, little to no talent, and the catchphrase 'Nyahaha!' he has been born again as a bottom-feeder boss destined to die in the tutorial! All that awaits him is his own grim end! However, after finding out that his beloved older sister is destined to die from illness, he makes her a promise: 'I'm going to wreck every unfair plot point in this damned world with my own two hands!'' Taking advantage of his vast knowledge of games, the man decides to rebel against the original scenario. The strongest secret boss, the boss from the saddest route… He sets out to meet all the bosses of the world. But what lies in store…?!'