Bögü Khan, the 'Sage-King', is the legendary founder-hero of the Uyghurs. In the origin-myth preserved by the Persian historian Juvaynī, a beam of light falls from heaven upon a mound that has risen between two trees by a river; the mound grows great and, opening, yields five infant boys reared by the people, the youngest and ablest of whom becomes Bögü Khan. A being of light in female form later visits him in his sleep and, over successive meetings at the White Mountain, promises him mastery of the world. The tale fuses the Turkic motifs of the sacred tree and the heaven-sent ray of sovereign light with the memory of the historical Uyghur khagans, and it is echoed in the Chinese account of the Yuan-shi.