According to most sources, the Yi were the first people to settle down in the Xilan region, founding their city and kingdom of Xilaroi.
The history of the Yi people can be divided into several periods, corresponding to the establishment and disestablishment of various dynasties, and the changes to the kingdom's name and policies in relation to this. The Xilaroi period is the first of these periods, covering roughly the first 400 years. It was followed by the Xiyi period.
The history of the Yi people during their first century of existence was relatively uneventful, and the kingdom was fairly isolated: dissent against the absolute monarch, the 'Silent Emperor', was punished harshly - and silently. The Yi lived to a fairly comfortable standard, however, and enjoyed significant civil liberties. Subsequent to the period known as the 'Tranquil Years', a century notable for its almost complete lack of significant historical events in the region, the Xiong tribes to the north became united. Known for their skill with the chariot, the Xiong took the opportunity that lay open to them and marched south to the fledgling kingdom of Xilaroi and the surrounding smaller kingdoms of Zhangat and Man. The Silent Emperor took the initiative: he treated with the Xiong tribes and made an alliance with them against the Zhangat King to the west.
A long, drawn-out war followed, with Xilaroi and the Xiong attacking the city-states and Zhangat. The city-states fell quickly, but conquering Zhangat proved by far a harder task: by its end, the Zhangat kingdom and the Yi army were both utterly broken. The Xiong found themselves in debt to the Yi as Yi intervention prevented a Xiong succession crisis and the first of the line of 'Young Khans' came to power.
Victory came at a cost, however, as the Zhangate cities had begun to house a severe pestilence. Carried on rat fleas, the 'Green Death', also known as the Vomiting Plague or Emesis Plague, soon spread throughout Xilan and decimated the region's population. Yi techniques mitigated the plague's effects slightly, however the effect was still devastating. It would take the kingdom almost a century to recover, and when it did, it emerged with a new name - 'Xiyi' - in honour of the brother of an Emperor, who had been a victim of the disease.