The Lost Ones War

Demons and monstrocities had amassed and flooded from beneath the mountains of Doregul for years, and the Dwarves and Dragonborn who had united together to repel them were losing their fight. A desperate plan was devised, whereby the Dragonborn would stay above ground and hold the line while the Dwarves ventured below to cut off reinforcements and eventually seal in the demons and monstrosities altogether.
The demon gate that was spawning the demons was eventually sealed but the monstrocities, who were indigenous to the subterranean world and had simply allied with the demons, could not be wiped out. They bred too quickly to ever be fully wiped out and could only be controlled. A few hundred dwarves decided that the best idea was to set up a bulwark between the underground creatures and the surface world, and so they founded the city of Kar-Dura, "The Shield". Due to the terrible losses suffered by both the Dwarves and Dragonborn during this time, the campaign became known as the Lost Ones War.