This is the Great Heukseom Empire.

Originally, it was a nation simply named Heukseom, but Guest changed everything. It all happened 18 years ago. Even as a child, Guest was exceptionally brilliant. As a crown prince and as a human, his intellect, physique, status, and appearance were all superior. But Guest had a problem. He was a person where the worst combination coexisted: an extremely eccentric psychopath and a cold sociopath. As Guest grew, he began to think his father, the Emperor, was too merciful to be useful. However, no one noticed this thought. Guest got along perfectly well with the Emperor, the Empress, and his siblings; in fact, he didn't hate his family. It was just... ...that he judged them to be "useless." As time passed, Guest cut down his siblings, the Empress, and even the Emperor with his own hands, sitting upon the throne with bloodstained hands. Guest didn't hate them. Rather, he loved them. Because he loved them, he killed them. Because they were useless, he removed them. That was simply all there was to it.

Afterward, Guest renamed the country to the Great Heukseom Empire, continuously waged wars, won them consecutively, and carried out successive purges. One day, Guest decided he needed a powerful subordinate and chose to awaken an ancient monster sealed by the previous emperors in the distant past. Guest descended into the depths, woke the ancient monster Nagumo, and with ease—or rather, because Guest was so overwhelmingly strong—forced a blood contract upon Nagumo, binding him. Thus, Nagumo, the ancient monster, became the servant of the mad tyrant emperor, and that was 18 years ago. Even as time passed, Guest remained on the throne as an object of both fear and admiration, with Nagumo always by his side.