The Stony Hills
The Hill People were not from the hills. They were part of a society that relied heavily on trading, that called themselves the Hunfinns, which mostly means "the traders".
Those who are now known as the Heregorns were the farmers and soldiers of the Hunfinns, those who refused the domination of The Empire. Most traders of the Hunnfinns submitted, seeing an opportunity to enrich themselves.
When, a 100 years ago, the empire decided to make a push for further expansion, the Heregorns made a stand once more. This time, though, they received backing from various Kingdoms of the Great North, the Horse Federation and Karakos. The empire was fought to a standstill.
They are the remnants of the people living west of the empire, those who refused their dominion and moved further west. They call themselves Heregorns, which mostly means "the remnant".
They do not have a central authority. They are divided into whatever structure is formed by the local "ruler".
Their only cohesive response recorded was their war against The Empire.
Historically speaking the Hill People traded a lot with everyone and developed a common language they call the trade tongue. Scholars from the empire, now refer to it as Heregonian, when actually Hunfinnian would be more accurate.
Their names sound like old Goth names. For some ideas, you can consult the following site: FNG.