Continuing the dungeon jam, but taking I’m forking off to the side a bit. Consider this an alternate path; I’ll explain rules consequences at the end. (John: feel free to continue this thread or not.)
The First Campaign into the Svartelheim was only a partial success. Key routes to the underground were mapped, but ultimately treachery within the Campaign, combined with the unanticipated encroachment of the Goblins, led to a defeat. Those first few heroes now low entombed below.
The elder dwarves, along with some surviving heroes, are now mentoring the youngest dwarves who must now be the front line in retaking the Svartelheim. They are not as hardened to battle, but they are passionate. In secret, by night, they train in the warrior arts until they are ready to enter the vast Underground. Meanwhile, distant cousins, foreign dwarves, and even foolhardy non-dwarven adventurers arrive and seek the mentors at the Secret Academy.
Slowly, most students will fall to the horrors of the Underground, but some survive to become yet more wise and more skilled. And so it is known that the Svartelheim will be reclaimed. It may take a hundred years, it may well take a thousand, but – inch by inch – the great city will be liberated and brought back to life.
So, this is suggesting a game that starts at – initially – more low-power. It would be more suited for running with an old-school retro D&D ruleset, perhaps like “Castles & Crusades” or “Labyrinth Lord”.