Gao Yun's ancestors were from the
Goguryeo royal house. When
Murong Huang defeated Goguryeo forces and temporarily occupied its capital
Hwando, he took many members of the Goguryeo royal house captive and moved them to Qingshan (青山, in modern
Jinzhou,
Liaoning), and their descendants became Former Yan and Later Yan subjects. As of 397, Gao Yun was a mid-level official in the administration of
Murong Bao (Emperor Huimin), when Murong Bao, under military pressure by
Northern Wei, abandoned the capital Zhongshan (中山, in modern
Baoding,
Hebei) and sought to take refuge at the old Former Yan capital of Longcheng (龍城, also in Jinzhou). On the way, his son
Murong Hui, resentful that his younger brother Murong Ce (慕容策) was made
crown prince over him, rebelled, and while Murong Bao was able to reach Longcheng, Murong Hui put Longcheng under siege. During one night, Gao Yun led about 100 soldiers and made a surprise attack on Murong Hui's camp, causing Murong Hui's army to collapse. Murong Hui fled to Zhongshan and was killed there. For Gao Yun's contributions, Murong Bao made him the Duke of Xiyang and adopted him as his own son, and so his name was changed to Murong Yun. It was around this time that he befriended another general,
Feng Ba, as Murong Yun was not looked upon favorably by others because he was a man of few words, but Feng Ba saw that he had wisdom and tolerance. Little is known about Murong Yun's life in the next few years. By 406, he was a major general under the reign of Murong Bao's cruel and arbitrary younger brother
Murong Xi (Emperor Zhaowen), when, at the instigation of Murong Xi's wife
Empress Fu Xunying, Murong Xi made a disastrous attack on Goguyreo in which many soldiers died not from battle but from exposure to the cold. Murong Yun himself suffered an arrow wound during the battle, and, because he feared Murong Xi's cruelty, used this as an excuse to resign his post and remain at home. In summer 407, Empress Fu died. Murong Xi mourned her so much that he ordered that a magnificent tomb be built for her. The officials were all forced to weep for Empress Fu, with those who could not shed tears punished severely, so they put spicy food in their mouths to stimulate tear production. Eventually, Murong Xi accompanied Empress Fu's funeral procession out of Longcheng. After Murong Xi left Longcheng, Feng Ba and his brother Feng Sufu (馮素弗), who had hidden themselves since Murong Xi had previously wanted to have them executed, conspired with their cousin Feng Wani (馮萬泥) to start a rebellion. They did so with the help of the general Zhang Xing (張興) and those who had previously conspired with the general Fu Jin (苻進) in a failed coup attempt earlier that year. Because Feng Ba was friendly with Murong Yun, he persuaded Murong Yun to become their leader, and they quickly captured the palace and closed the city gates. Murong Yun was declared the Heavenly King. Murong Xi returned to Longcheng and settled in outside his estate of Longteng Chateau (龍騰苑), preparing an assault on the city. At this time, the imperial guard soldier Chu Tou (褚頭) fled to him and informed him that the imperial guards were ready to turn against Murong Yun as soon as Murong Xi attacked. However, for reasons unknown, Murong Xi panicked at this news and fled. His general, Murong Ba (慕容拔), tried to maintain the assault against Longcheng and was initially successful, but as the troops began to realize that Murong Xi had fled, they collapsed, and Murong Ba was killed by Feng Ba's soldiers. Later that day, Murong Xi was found, wearing civilian clothes, in a forest, and he was captured and delivered to Murong Yun. Murong Yun personally read him his crimes, and then beheaded him and his sons. ==Reign==