Witten's Tears

The Battle of Witten's Tears is the longest and bloodiest of 14 years of long and bloody war. But the last of Torg's line, his young grandson, and Benjamin II of Issur die on each other's blades, among some 50000 others. The Torgs surrender and vow to never cross the Pellinores or the Issur river. The south returns home.

It was impossible for the witches and even the new Vitalists to keep up with blessing the dead and sanctifying ground for so many years of war, and by time of Witten's Tears, everyone was so exhausted they just basically packed up and left.   So in fact in the centuries since, it's been easy for the Torgs to keep their promise, as broad swaths of the lands close to the mountains are now essentially haunted and infested with undead. Huge parts of the empire are completely uninhabitable for those who farm or work the land in any way - or if not uninabitable, at least undesirable; no one wants to till the land when an old hand might just reach out and grab you.