The destruction of Tor Darrow was, according to most Historical accounts, the catalyst for the end of the Witch War.
The destruction of the kingdom of Tor Darrow was actually long in the works stretching back into the history of the Gnomish people far beyond the Witch War. The bitter rivalry between the Gizaryl and the Berkeron, which first errupted in the Golem Wars, had continued for nearly three hundred years.
The Berkeron were losing the Golem Wars and decided to tap into Necromantic energy to improve their odds. Rather than implant the golems with a core of arcane energy to power them, the Berkeron Artificers bound living souls. The new evolution allowed their golems to react more brutally to the war - able to adapt to changing situations and learn from their mistakes rather than following pre-written instructions the way most golems were made. By enslaving the dead to their Golems, the Berkeron angered both the Gizaryl and the Ulevar who had attempted to remain outside of the conflict until this revelation.
The combined forces of the Gizaryl and the Ulevar were not only a match for their Berkeron cousins, but drove them back to their mountain strongholds. Locked behind their thick iron-clad gates, the Bereron hatched a devious plan of revenge that they thought would end the war once and for all. While the combined forces of Gizaryl and Ulevar were busy keeping their main gates closed and presumably the danger contained, the Magi of the Berkeron tapped into the deepest of the arcane paths and drew out more power than any cabal had before. They called more and more power and channeled it into an arcane core - one used to power their largest golems. Once the core was empowered with more energy than anyone thought it could, they teleported it to the city of Tor Darrow and Gibblenook - a city of the Ulevar in an area that would eventually be called the Provinces.
As expected, the shock of teleportation was more than the core could handle and they exploded as soon as they arrived. Tor Darrow was flattened by an explosion so great that the flash of light burned a shadow of their largest tower into the ground; a reminder etched into the bedrock. Only a single tower remained at their river port - though it was never known as to how it survived. The impact of so many Gizaryl lives being snuffed out in such an explosion was echoed as Gibblenook, the Ulevar's second capital, was removed from the lands leaving only a large crater in its wake.
The remaining gnomes of the Gizaryl and Ulevar were united in their need for revenge. They would make absolutely certain that the Berkeron would not harm another of their people.