Age of Formation
This was a turbulent time where, in the wake of the fallen empire, noble bloodlines fought viciously for power and control, of what would become the 5 Kingdoms. This led to much suffering among the people, whose frustration led to more bloodshed and unrest.
After the empire, the eastern part had at first reverted to a multitude of independent fiefdoms, as every count declared himself king. Fairly quickly, though, the five major kingdoms were formed based on pre-imperial tribes, marriages and trade alliances. Once these five rough kingdoms were defined, the squabbles turned internal within them, as the most powerful families battled for the respective thrones. And still, along the edges, smaller lords would break into skirmishes over rights to a hill or a patch of trees. The empire was gone, but there was no peace in the lands.
It was the formation of a secret alliance which would solidify the current borders. They were able to do so, because it was an army controlled by noble leaders from each of the kingdoms, banded together, eventually also utilizing the Arcanum, which had otherwise remained neutral up until that point. At first, they were able to operate unseen, in the midst of the countless factions still warring for power at the time.
The common people suffered more for lack of clear leadership, and in some places reverted to old tribal structures, and worshiping of ancient gods. This degradation of civilization was seen as a threat by many of the stronger nobles, who grew desperate to end the constant squabbling once and for all.
Once this alliance was strong enough, they used agents to get the common population turned against those who had an air of superiority about them, like the cultured elder races, the elves and dragonborn. Very quickly, the human and dwarven populations flocked to the cause, feeling that these self-important races were conspiring to keep themselves from building a new civilization on the ruins of the fallen empire. It was this movement that eventually evolved into the Cleansing.