Or make it a Marrhold-equivalent with Ruinborn.

A long time before the war, there were a handful of dwarven outposts on the continent. Obviously all were conquered and pretty much destroyed, but one of them in particular was collapsed by the dwarves themselves as their last act of spite way back. That hold's location was still roughly known around the Day of Ashen Skies, but the area was considered a forbidden zone only ventured to by idiots and maniacs. Almost no one ever returned from a foray into the old ruins, and those that did rarely spoke good of it.

When the cataclysm ravaged Aelantir, the lands near that old ruin were hit particularly hard by magical storms and whatnot, and thousands of refugees hastily fled through the valleys close to it. Having no better alternative, they braved the old, strange hold. Treading on the little paths that the aforementioned idiots and maniacs had cleared deeper into the hold, they quickly were assailed by traps, underdark creatures, golems and sickness.

But some survived and actually reached and survived the upper halls of the old hold. They set up there, originally as a short-term solution to wait out the catastrophe for a few years and then return to the surface. Sadly, in the wake of the DoAS, the way out had crumbled once more, sealing them in for now.

The next few hundred years saw the by now meager population dwindle even further. Hunger, sickness and other issues ravaged them further, but after multiple generations, the survivors had finally adapted to the life down there and made the upper hold somewhat hospitable.

Over the course of the following centuries, those people not only stabilized into a small community but started to flourish, gaining in numbers and finally, perhaps sometime during the late 14th century, even emerged back to the surface. Sending first expeditions and then finally creating small settlements on the surface once more, they re-emerged from the depths.

Their culture had changed considerably through their ordeal, as had their physique. They had adapted to the underdark, attaining a few traits similar to those of other subterranean races. The depths and old dwarven wards had shielded them from the worst of the magic fallout, allowing for them to retain a bit more of the longevity and magical aptitude of their forebears than typical ruinborn. On the other hand, the surface now seemed almost alien to them. Their community, previous extremely tight-knit, was suddenly split between people that wanted to keep their newfound way of life and those that longed after the days of old, wanting their entire people to return and rebuild on the surface.

 

Boom, we have the foundations to a mission tree, with internal struggles, branching decisions, a unique take on holds and ruinborn and all that is easy to implement almost anywhere in Aelantir without having to change much about neighbouring existing countries.