How Zhao Xian, the second last Emperor of the Southern Song Dynasty ended up had remained a myth for centuries. As there were hardly any historical records clearly documented when, where and how he did, controversies and even legendary talks were generated around this topic among scholars of the Ming and the Qing Dynasties. The most peculiar one claimed him the biological father of the Mongolian Ukhaantu Khan, Emperor Shundi of the Yuan Dynasty. This lecture looks into the historical documents in both Chinese and Tibetan to recover the whole story in which a child emperor of the Song firstly became the Duke Yingguo of the Yuan and later the monk Lha btsun of the Sakya School of Tibetan Buddhism. Relevant legends are also discussed in terms of their origin, development and authenticity.