By this point, Heba Yue, in alliance with Houmochen Yue, controlled almost all of the western provinces. However, one provincial governor, Cao Ni (曹泥), the governor of Ling Province (靈州, roughly modern Yinchuan, Ningxia), was aligned with Gao Huan and refused to follow Heba's orders. Heba Yue sent his assistant Zhao Gui (趙貴) to Xia Province to request Yuwen's opinions, and Yuwen, believing that Houmochen was unreliable, advised against an attack on Cao and suggested instead that Houmochen be attacked. Heba refused—not realizing that by this point, Gao's messengers had persuaded Houmochen to act against him. Heba and Houmochen rendezvoused at Gaoping (高平, in modern Guyuan, Ningxia), and then headed north against Cao—but as they advanced, Houmochen tricked Heba into coming to his camp for discussions, and then had his son-in-law Yuan Hongjing (元洪景) assassinate Heba. Initially, Heba's army was surprised and intimidated, but Houmochen, instead of taking over Heba's army, panicked and fled to Shuiluo (水洛, in modern Pingliang, Gansu), while Heba's army, without a central commander, withdrew to Pingliang (平涼, also in modern Pingliang). After some internal discussions, the army commanders decided to offer the command to Yuwen Tai, and they sent Du Shuozhou (杜朔周, later changed his name to Helian Da (赫連達)) to Xia Province to summon Yuwen Tai. Yuwen agreed, and Du and he quickly headed back toward Heba's army. (On the way, they encountered Gao's general Hou Jing, whom Gao had sent to try to take over Heba's army; Hou, surprised, withdrew to Gao's territory quickly.)

Emperor Xiaowu, hearing of Heba's death, sent Yuan Pi (元毗) to summon both Yuwen and Houmochen to the Luoyang. Houmochen outright refused, and Yuwen persuaded Emperor Xiaowu to allow him to remain in command. Emperor Xiaowu agreed. Yuwen next sent a letter to Houmochen to rebuke him, and when Houmochen did not answer, prepared to launch an attack on Houmochen. He advanced quickly on Shuiluo, and Houmochen withdrew to Lüeyang (略陽, in modern Tianshui, Gansu), and then to Shanggui (上邽, also in modern Tianshui). He then further withdrew from Shanggui, and Shanggui surrendered to Yuwen. He decided to try to flee to Cao's territory, but on the way, believing that Yuwen's forces were close, committed suicide.