The northlands were conquered by the Elabonians in the early days of the Empire. Elabonian people colonized the lands and appropriated some native culture, including the worship of the god Biton. The danger of the barbarian Trokmoi made life in the northlands a constant trial.
Over the centuries, the northlanders began to develop their own cultural identity, while still identifying as Elabonian. This situation was exacerbated when, during a new wave of fighting by the Trokmoi, the Empire sealed off the most important passes between the north and the Imperial heartland. In the reign of Emperor Hildor III, the Empire ceased to send aid to or collect taxes from the north, and several northern barons began to style themselves as Prince, or in some cases King.
Hildor's successor Crebbig the Magnificent attempted to forcibly bring the northlands back into the Empire, but his defeat seemed to assure a permanent independence for the region.
"North Elabon" is never referred to as such in-universe. The article is titled for convenience.