The Great Northern Desert is a high-altitude, bare-rock desert plateau, similar to the Gobi Desert in RL Mongolia. It is a desolate place of extreme seasonal shifts, with scorching summers and long, frigid winters. Near the northern limits it transitions smoothly into the tundra of the Northern Wastes; ironically there is actually more visible life at this end of the desert, because the summers are cool enough that lichens are able to survive (which in turn allows for insects, migratory birds, and even some caribou).
Few people have any reason to want to visit the Great Northern Desert. The ancestors of the Lom Shi'Un Dynasty traveled through it during their search for a passage into the Great Barrier Range; they eventually found the Devil's Pass and made their way through to the Giantdowns, but the crossing was hard and without magic none of them might have survived.
Today, the desert serves as an effective barrier to any army attempting to pass out of the Giantdowns; even if they could brave the Devil's Pass, the difficulties of supplying an army with food and water while it crossed the desert would be prohibitive. This makes it all the more crucial for Nasoj to take Metamor Valley in order to advance his territorial ambitions; there is literally nowhere else for him to go.
It is rumored that one of the Titans is imprisoned in the Great Northern Desert, but the location is shrouded from all forms of divination magic and has yet to be found.