The Ashen Debt, 1020 years after Malnaar's imprisonment, during the height of the dwarven Golden Age.
Karn'thûreth, the Ashen Debt, came up out of a small tear beneath the dwarven capital of Emberdeep (Mor-Udul) and unleashed its fury upon the city. It is one of the karn'tharaël, the aberrant wound-things of the same kind as Karn'tharaël, the First Wound: lesser than the First Wound, bound to a lesser tear, and still more than the dwarves could hold.
The city, renowned for its intricate stonework and vast underground halls, was reduced to rubble in a matter of hours. Like all its kind, the thing bound what it held, so that the defenders could not strike it without striking their own. Survivors fled wherever they could.
The king of Emberdeep died in the halls. A runer named Tharn, known to humans as Thrain Ironfist, led the survivors out. He lost his right hand doing it, cutting it off himself to get free of the thing's grip.
Karn'thûreth never left. It is still anchored to its tear beneath the ruins and siphons mana through it. The great forges of Emberdeep are dead and cold, and yet they still work: their fire is gone, replaced by pulled mana. Metal there is not heated but drawn. This cold draw is why it was possible, much later, to forge and seal dragonsteel in the ruins at all.
With their capital obliterated, the dwarves relocated to Stoneculling (Khaz-Kuldun), which became the new heart of their civilization, and the Stonecouncil made Thrain its First. The fall left a lasting sense of vulnerability: the Empire poured its strength into fortifying the new capital and into knowledge of ancient evils. That same inward turn thinned the guard on the Gul'durak, whose murder some eighty years later freed Malnaar and broke the world (see The Reawakening of Malnaar).
The loss of Emberdeep's cultural and historical treasures was deeply felt. In Durûm the city is still named for what it became: Mor-Udul, the banked heart, where it had once been Brom-Udul, the forge-heart.
Older records name the destroyer Behemoth. That is an error of the human chronicles: Behemoth is the Avatar of Time, and had no part in the fall.