According to this Universe,
3000 yrs ago, A Demon King Ruled the Earth by defeating all the powerful mages and Kings present at the time on the Earth.
But, Some Elite Mages who survived, together gets successful in sealing the Demon King in exchange of their own lives.
While dying one of the mages,who had light magic get a glimpse of future, and say something that made that era people happy and worried at the same time.
He said," The seal is not so strong to seal the Demon King for eternity, but don't worry, when the Demon King will release A Hero with great magic and all types of Magic will be born in the world and kill the Demon King.So, believe in the Hero.
"THE END"
The lady said as she closes the book.