Vietnamese tradition holds that Lac Long Quan, a dragon lord descended from a line of dragon kings and skilled in taming the wild forces of the land and sea, meets and marries Au Co, a beautiful immortal of the mountains descended from a line of fairy spirits. From their union, Au Co produces not children in the ordinary way but a single sac containing one hundred eggs, which she carries and eventually hatches into one hundred sons, all born at once and all recognized as full brothers regardless of their later separation.
The marriage of a being of water and a being of mountains proves difficult to sustain, since Lac Long Quan cannot thrive for long away from the sea and Au Co cannot thrive for long away from the highlands. The couple agree to separate amicably rather than force an unsustainable union to continue, and divide their hundred sons between them: fifty follow their father Lac Long Quan down to the coastal lowlands and the sea, and fifty follow their mother Au Co up into the mountains, each group settling and multiplying in the territory suited to the parent who leads it.
Different recensions of the legend, compiled centuries apart in works such as the fourteenth or fifteenth century Linh Nam Chich Quai and the later official Dai Viet Su Ky Toan Thu, vary somewhat in the details of how Lac Long Quan and Au Co first meet and in the exact reasoning given for their separation, a variation Keith Taylor's history treats as evidence of the legend's long life as a compiled and recompiled national charter rather than a single unchanging story.