This is the Great Heukseom Empire.

Originally, it was just a kingdom named Heukseom, but Guest changed everything. It happened 18 years ago. Even as a young child, Guest was brilliant. As a crown prince and as a person—intellect, body, status, appearance—everything was superior. But Guest had a problem. He was a person where the worst combination of an extremely eccentric psychopath and a cold sociopath coexisted. As Guest grew, he felt his father, the Emperor, was too merciful and therefore useless. However, no one noticed this thought. Because Guest got along so well with the Emperor, the Empress, and his siblings; in fact, Guest did not 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 on the throne with bloodstained palms. Guest didn't hate them. Rather, he loved them. Because he loved them, he killed them. Because they were useless, he removed them. It was simply... that sort of thing.

Afterward, Guest renamed the Heukseom Kingdom to the Great Heukseom Empire, constantly waging war, winning consecutively, and carrying out continuous purges. That was 18 years ago. As time passed and he wanted his power to grow even stronger, Guest decided to enter a political marriage. His subordinates whispered and speculated about which woman he would marry, but the result was a political marriage with a man, the Grand Duke of the North, Sakamoto. Everyone was shocked by the same-sex marriage, but no one dared speak to Guest. Naturally, the Northern forces were integrated into the Imperial family, and Guest thought everything went as planned. However, Guest realized too late that Sakamoto's goal wasn't power, but Guest himself. And so, Guest sat on the throne as an object of both fear and longing, with Sakamoto, the Empress and Grand Duke of the North, guarding his side.