Lore: Immerdwelm

The image above is a cross-section of Immerdwelm. The central pillar does not bisect Immerdwelm's great cavern; rather, the cavern forms a ring around it. Likewise, the citadel is, in fact, surrounded on all sides by the new city. Not pictured is another nearby hill, carved into a walled harbor, which functions to this day as the city's port. (This diagram is only very roughly to scale.)  

At the time of its destruction, Immerdwelm was the oldest extant city on the Periandic Peninsula. Ground was broken by dwarven settlers millennia prior, with the permission of the region's then-halfling rulers, who leased the land in perpetuity in exchange for a portion of its mineral wealth.

At this point, dragonkind had not yet suffered its curse at the hands of Xogoth's elven cult. When this atrocity came to pass, the vast majority of dragons were transformed into the weaker and shorter-lived Draegar. With swift vengeance, Draegar and dragons, once protectors of the region, established a brutal imperial dominion over it. Immerdwelm was put to siege, and eventually was absorbed, its people forced into servitude.

When the draconic empire at last fell, the dwarves remaining in Immerdwelm were joined by an influx of colonists from their Ironheart brethren, and together they founded Clan Hill-Breaker. Their new polity would gradually expand its control over the peninsula's northeast, shaping the landscape as their ancestors did with Immerdwelm.

In the ninth century, S.R., conflict would escalate between Clan Hill-Breaker and the rising Kingdom of Periandor. A Bitter war followed, culminating in the Perianders' encirclement of Immerdwelm. Daunted by the task of seizing the city, King Andro instead flooded it.

The greatest part of old Immerdwelm lay beneath the ground, ensconced in high bluffs overlooking the sea. As the ocean filled Immerdwelm, the lower portion of its great cavern, and all of the adjoining tunnels, chambers, and mines, were submerged. The scale of this catastrophe forced the surviving people of Immerdwelm to the surface, where they were captured or slaughtered by their besiegers.

With the lower city uninhabitable, the freshly minted Duke of Immerdwelm claimed the upper reaches of the citadel as his own seat, while allowing his human subjects, and handfuls of dwarven survivors, to settle around its base, establishing a new city under his rule.