As the Hunnu weakened, a nomadic tribe from
eastern Mongolia and Manchuria whom they had conquered early in their 400-year
rule rebelled. Known as the Xianbei, they quickly gained control over former
Hunnu lands and resumed assaults on Chinese territory, eventually forcing the
collapse of the Han Dynasty – and a unified China- in 220 CE.
disintegration that followed, the southern branch of the Xianbei, known as the
Toba, eventually turned sedentary themselves and became Sinicized, adopting
Chinese ways. They established the Northern Wei Dynasty (386-533 CE), one of
the Three Kingdoms in this tumultuous period of the Chinese history. The
dynasty controlled a vast region of the northern Xuanbei, now known as Rouran,
gained control of the Mongolian heartland and attacked their southern cousins,
they were forced to follow Chinese custom and reconstruct the Great Wall.
Resource: Carl Robinson "Mongolia Nomad Empire of Eternal Blue Sky"