Northern Chanyu
The Northern Chanyu (Chinese: 北單于; pinyin: Běi Chányú; Wade–Giles: Pei Ch'anyü, reigned 89–91) was an unnamed and obscure chanyu or ruler of the Xiongnu who lived in the 1st century CE. In February 91, he was defeated by Geng Kui during the Battle of the Altai Mountains, on an expedition sent by Dou Xian. His younger brother Yuchujian Chanyu (reigned 91–93) was his sole heir, but was killed by generals Ren Shang and Wang Fu in 93. The ideal situation on the frontier was to have a non-Chinese ruler so powerful within his own lands that his orders were obeyed but so dependent on Chinese goodwill, or vulnerable to Chinese threats, that he kept his people from troubling imperial territory. By destroying the Northern Shanyu, the Han removed a potential client and found itself faced with the incoherent but spreading power of the Xianbi, while the Southern regime was overwhelmed by its new responsibilities.
The Northern Chanyu (Chinese: 北單于; pinyin: Běi Chányú; Wade–Giles: Pei Ch'anyü, reigned 89–91) was an unnamed and obscure chanyu or ruler of the Xiongnu who lived in the 1st century CE.
In February 91, he was defeated by Geng Kui during the Battle of the Altai Mountains, on an expedition sent by Dou Xian. His younger brother Yuchujian Chanyu (reigned 91–93) was his sole heir, but was killed by generals Ren Shang and Wang Fu in 93.
The ideal situation on the frontier was to have a non-Chinese ruler so powerful within his own lands that his orders were obeyed but so dependent on Chinese goodwill, or vulnerable to Chinese threats, that he kept his people from troubling imperial territory. By destroying the Northern Shanyu, the Han removed a potential client and found itself faced with the incoherent but spreading power of the Xianbi, while the Southern regime was overwhelmed by its new responsibilities. So the empire destroyed a weak and all but suppliant enemy for the benefit of a junior ally who could not make good use of the victory, to the ultimate profit of a far more dangerous enemy.
According to the Book of Wei, the remnants of Northern Chanyu's tribe, whom Lev Gumilyov termed "Weak Xiongnu", settled, as Yueban (悅般), near Kucha and Wusun; while the rest fled across the Altai mountains towards Kangju.

Footnotes
References
Barfield, Thomas (1989), The Perilous Frontier: Nomadic Empires and China, Basil Blackwell
Bichurin N.Ya., "Collection of information on peoples in Central Asia in ancient times", vol. 1, Sankt Petersburg, 1851, reprint Moscow-Leningrad, 1950
Chang, Chun-shu (2007), The Rise of the Chinese Empire 1, The University of Michigan Press
Cosmo, Nicola Di (2002), Ancient China and Its Enemies, Cambridge University Press
Cosmo, Nicola di (2009), Military Culture in Imperial China, Harvard University Press
Crespigny, Rafe de (2007), A Biographical Dictionary of Later Han to the Three Kingdoms, Brill
Loewe, Michael (2000), A Biographical Dictionary of the Qin, Former Han, and Xin Periods, Brill
Taskin B.S., "Materials on Sünnu history", Science, Moscow, 1968, p. 31 (In Russian)
Whiting, Marvin C. (2002), Imperial Chinese Military History, Writers Club Press