The Northern Tribes are a group of people who live in the lands far north of The Empire. Their society consists of small tribes scattered throughout their territory. The tribes are fairly modern, as they are shown to have buildings and steam working shops. Its largest city (and possibly the capital) served as a powerful stronghold, with Numa Seika being a major hero within their military and the prince of the north.
The tribes were attacked by the Empire due to the threat they posed by their rapidly-rising military strength. An estimated 400,000 inhabitants were killed when general Esdeath buried them alive during this military campaign and broke their hero's spirit before killing him. Esdeath's army is currently stationed there to continue to watch over its people to prevent future uprisings.
A tribe of Danger Beast hunters called the Partas were completely wiped out by one hostile tribe that crossed the border into the Northern Frontier lands of the Empire. The chief of this clan was Esdeath's Father.
During the Revolutionary uprising, after Esdeath had left to prevent the rebels' further advancement towards the capital, a group of northerners laid in wait in the city for their opportunity to storm the castle and take revenge against the empire for the death of Numa Seika and the atrocities they've caused by taking the young emperor's head. Upon their assault, the northerners were easily outmatched and slaughtered by Suzuka and Kurome, who puppeteered Run, Natala and Doya, while Wave chased down one of the members and arrested him, noting that the group was quite skilled, managing to make it all the way to the empire's core without a Teigu at their disposal.
Later upon the conclusion of the Empire's Revolutionary Civil War, it had been confirmed that due to a majority of their population being massacred and the majority of their Millitary being completely slaughtered by General Esdeath and her army, the Northern tribes as a whole have now been rendered extinct and the nations itself is now defunct and no longer possesses any threat to the new nation being built upon the Empire's fall.