Orcish Invasion of the North

Led from their eastern homelands en masse by power-hungry wizards who thought to control this horde of chaos, masses of orcs swarmed into the Waterdhavian Empire, razing countless villages and ending the period of unity between the North's humans and dwarves. Villages were razed, entire Dwarven strongholds put to the sword and countless slain. The last Waterdhavian Emperor and his son and heir perished on the battlefield together, prompting Neverwinter and Waterdeep to end their union and turn inward.
Although almost all of their holdings outside their walls burned, but these defensive measures were enough for the cities to wait out the storm until the orcs lost their leadership and returned east.