Opposite to the Alive Plains were the Dead Plains, and unlike the Alive Plains, which were ruled by three hegemonies together, the Dead Plains were ruled by only one power or one Dark Being, to be exact, the King of Dead!
The distribution of the Alive and Dead Plains solely depended on the strength of both factions. But any intelligent being is comparative by nature, especially if the comparison is with the arch-enemy!
That's why both sides maintained a delicate equilibrium and controlled half of the Epic Plains each and didn't let each other further increase their territories.
However, for Dark Beings, who basically live and grow by feeding on the living, peace was the least of the thing they wanted. That's why they always try to invade the Alive Plains no matter what and slaughter everything that has life.
It was the biggest reason the three hegemonies always needed manpower and strong forces to protect their borders.