The Northern Empire is the heart of Kyros the Overlord's Empire and the seat of her power.
What is now known as the Northern Empire was, a bit more than a century ago, known as the Northern Kingdom, until it fell to Kyros in the penultimate conquest. During its time, to those on the receiving end of it, it was known as the Great Northern War of the Overlord, but Tunon wrote a law that outlawed the use of the name (on pain of clipped tongue, and mangled hands if literate) and that name has since been forgotten. To those not trained in the Vellum Citadel or by Fatebinders who are quite jaded when far from Tunon, it is known more simply as the Northern Conquest.
Led by Blood Ruin, then Archon of War, the Overlord's armies expected the North to fall within a year. They did not count on the tactical brilliance of general Graven Ashe. Outnumbered and cut off from supplies, the great general led his troops in a holding action that stymied all of Blood Ruin's plans. It was during this desperate defense that Graven Ashe began to manifest the abilities that would allow him to challenge Blood Ruin for the mantle of Archon of War.
After defeating Blood Ruin, Graven Ashe was defeated by several of Kyros' Archons and brought before the Overlord in chains. No one knows what Kyros said to the general that day, but Ashe emerged a loyal servant of the Overlord. With his surrender, the Northern legions bent knee to the Overlord as well, and the last great Empire of Terratus fell to Kyros' rule. The remnants of Graven Ashe's soldiers - those who survived the final battle with Blood Ruin - became the core of his Disfavored legion.
The Northern Kingdom was a heavily tribal culture that Ashe and his militia martyred themselves to defend. However, the bonds of blood between kith and kin resulted in a tightly-knit society capable of achieving great feats (as the Disfavored show)... And incredible atrocities against people they consider inferior (again, Disfavored).