Demons, people have known about their existence hundreds of years ago. Demons are different from humans. Humans only live for a few decades, but demons can live for hundreds or even thousands of years. In order to live forever, the noble sons of the royal family did not hesitate to catch monsters at the cost of people's lives and use their flesh and blood to make medicine. The emperor even created an organization called "Monster Hunters" in the name of eliminating harm for the people.
Concept: A story about a salty fish author's whim