The 12 Royal Families had made a pact of not invading in the other eight plains. This pact was actually imposed by the Demon-slaying Hall in order to keep the 12 Royal Families within bounds.
After all, should the Great Land be under control of only the 12 Royal Families, other forces could hardly grow. Many ambitious people would feel they had no chance of advancement and their eagerness of making an effort would diminish.
Granted, the 12 Royal Families could not invade the other eight plains. But it didn't mean that the eight plains couldn't fight each other.
Once, there was a family that unified three plains. The family developed and soared up fast. Ultimately, the family conquered a piece of land in the Central Plains and grew to be one of the overlords.
That family was now known as the Pavilion of Blossom!
