Who Killed The Hero?

There was a prophet who declared, “A hero who will defeat the Demon King will appear from this village.” The village believed in the prophecy because there was a boy who was skilled in swordsmanship and magic. Eight years later, the boy became a hero and defeated the Demon King. However, at the same time, he became a person who would never return. Why did the hero die? Four years after his death, as his achievements were being compiled into literature, the truth behind his death was about to be revealed.