Firehold Megadungeon

This might turn into yet another project I don't have time to write or run, but it's been on my mind lately, so I might as well put it here. (Long time readers might recall my plans for a Carcosan megadungeon; apart from a single session nested in my long-running D&D game, I haven't had a chance to initiate it. Although there is always the future.)

When the dwarves fled from bondage under the tyrannical rule of the giants, they settled across the Sorrowfell Plains, most of them settling into cities and towns that are now familiar to the folk of the world (such as Khuragzar or Sorgforge), but one group of now-forgotten clans settled beneath one of the mountains of the world. 

They called this place "Firehold."

Industrious and versed in runecraft, they made wonders unknown even to other dwarves. The secret of their success was the beating heart of the mountain itself: a uranium deposit beneath the mountain, creating a naturally-occurring fission reactor. When they initially found it, they realized only that it generated heat and made the miners sick. (Although their superior dwarven constitutions protected them somewhat, many of them still died, but they were given heroes' burials for their sacrifice to the clan.) But the industrious dwarves soon learned how to harness this energy for their own purposes, building containment units and enriching centers and steam engines. They were the most advanced civilization on Khaldun.

But no one has ever heard of them.

When the lands of Men fell to corruption and devil-worship, the dwarves consulted their histories and feared what they saw coming: another cycle of darkness and subjugation beneath the whips and chains of another oppressor. So they did the most sensible thing they could consider: they withdrew. They secured their gardens and redoubled their mining efforts and ensured that they had supplies for centuries — millennia, even — and then they sealed their highways with rune-magic.

This was their folly.

The corruption of the surface empires did not last long before heroes rose to stop it, and before long it was washed away. But the lost dwarves did not know of the turning of ages; they merely waited for a sign, some manner of augury to return them to the surface.

They never received it.

Whether the gods did not speak to them or they merely misinterpreted the signs, the old priests refused to risk the safety of their clans, and so the dwarves stayed underground in their secure shelter.

But the deep dwarves were builders, not soldiers. One by one, monsters emerged from below, and one by one, districts of the city fell. The deep dwarves were forced to retreat, refugees in their own city, withdrawing to the safety of centralized, defensible locations. Now, the last of the dwimmer dwarves take refuge in the quays and the foreign quarter, praying for a sign to save their civilization.

And all the while, the danger grows beneath their feet, for no one has tended the reactor in generations. If the reactor burns hotter than the reservoir replenishes, or if some tunneling wurm or ambitious underdweller should damage it, it could cause a meltdown or an explosion, contaminating the ground water or spreading radioactive fallout across the Sorrowfell Plains.

In addition to references in ancient archives, there is one other hint as to the location of lost Firehold: the rocs that lair upon the mountaintop build their nest over the exhaust port, so that the steam keeps their eggs warm while they are in flight.

Rules Notes:

A player character who delves into the right archive or uses the right divination spell or asks the right questions might find and unlock Firehold as an adventuring location. A lost megadungeon of advanced dwarven technology, it has long since been overrun by monsters, but is a palace of wonders. (In addition to the standard treasure, the dwarves of Firehold potentially hold lost secrets of arcane, historical, and technological varieties.)

Anyone who finds lost Firehold also unlocks a new playable variety of dwarves: called dwimmer dwarves or bone dwarves in my notes, they have a facility with rune-crafting and artifact manufacture that most modern dwarves lack. (In old-school games, they would be able to craft magic items and undertake other sorts of spell research. In 5e-ish games, they have innate transmutation spells, and are best suited for the wizard or artificer classes.)