During the battle between the heaven and the demon world, Michael, the King of Light, led the heavenly army to defeat the demon world. The demon king Lucifer was missing and the demon world was in turmoil. Lucifer's subordinates led the remaining demon army to hide everywhere.
Cain, the ancestor of the blood race, was injured by Michael. His physical and mental age was greatly reduced, and he became, well, a little shota.
So Cain embarked on the road to find the king.