There used to be 8 Dwarven kingdoms. and the Vicehandler at the time when the drow were accepted did so on the agreement that the drow had to earn their way in Dwarvish society as all dwarves did, through merit. And under no circumstance were they allowed to settle within the borders of the capital

The Drou, tired and willing to accept this new chance accepted. They had little enclaves within Dwarven society and some through their crafting or skill in battle even came to be respected. And in a few extremely rare cases were allowed to form a Clan that could participate in politics. Overall this was going well.

In most of the Dwarven kingdoms, the two races found some amount of peace and camaraderie. But in the last, things were amiss.

The eighth kingdom was a poor land. In minerals and what meagre crops could be obtained. And so tensions with the new Drou populations were almost immediate. Rather than being treated as equals or even second-class citizens the drou were shunned and forced to work the hardest deadliest jobs. Many were made fodder for the expansion of the eighth kingdom. Blood scouts, they were called. If they came back then that was a tunnel ready for conquest, if they didn’t then it was one where a fight was to be had.

Many were subjected to slavery in all but name and thoroughly broken. The advisors and inspectors of the Vicehandler were bought off with gifts and coin to look the other way to the injustice. After nearly two hundred years of this, there was a massive revolt by the Drou population in the eighth kingdom slaughtering the rulers and forcing any and all prisoners out to the tunnels, unarmed and vulnerable as so many of the Drou had been.

The other 7 kingdoms ignorant of the injustice in the 8th were horrified beyond measure. They did not see slaves breaking their own chains but a repeat of their past conflicts with outsiders.
Drou in the Dwarven lands came under great scrutiny and many were drafted into the army to prove their loyalty. Those who didn’t or couldn’t found themselves either sneaking away to the new Drou kingdom or being held in ever more restrictive ghettos.

Since then the war has raged ever since. The Drou population in dwarven lands ever shrinking.