There is a legend that is narrated to the first rulers of Vietnam, dated to 2838-2698 BC which corresponds to the Phung Nguyen culture traced to a small settlement in Bac Bo (North Vietnam). According to this legend, De Minh, the great grand son of the Chinese emperor Shen Nung, married an Immortal in Honan Province. Their youngest son, Loc Tuc, was appointed to rule over Xich Quay which included, apart from territory in China, North Vietnam. Loc Tuc's son then married the daughter of King De Lai, Princess Au Co.
They had 100 sons, and, as per the prevailing societal custom of the Vietnamese which was matriarchal, the 100 sons were under the care of Au Co, while her husband lived with his own mother. Suddenly one day Au Co's husband announced to her that he belonged to the race of dragons and she to the race of the Immortals and that henceforth they could not live together. He thereafter moved south with his 50 sons; his wife moved to the hills in China with the other 50, thus ushering a shift in the social culture of Vietnam, from matriarchal to patriarchal.
This separation is said to have been the first divorce in Vietnam. It is conjectured that the hundred sons who were thus “scattered across the south of China were the Pai Yueh (Hundred Vietnamese)”. The sons of Lac Long Quan who inherited the kingdom took the title of "Hung Vuong" ("Brave King") and founded the Hong Bang Dynasty, which lasted from 2879 BC to 258 BC. The name of the kingdom was changed to Van Lang, a tribal totem. The kingdom went through a series of changes with turbulent history, having changed hands a number of times and Au Viet came to existence with Cao Bang as its capital. This legend is linked to present day Cao Bang, which is at the border with China.