This is a book, a book that lies underground. Ancient stories are recorded in ancient books.
There are demon clans from the demon kingdom and the ancient king clan, each with 3/1 territory. So where is the other 3/1 territory? Why don't the powerful clans invade that land. It is recorded in the book that the world can only be opened by the children of destiny. A world where demon kings coexist. The story unfolds from here...