Ethnic map of Nepal, source: CIA
Sikkim (Bremo shong, or Brejong, meaning Valley of Rice; Tib. 'Bras mo gshong; 'Bras ljongs) is a territory located between Nepal and Bhutan, settled by Tibetans in the 17th century. The first religious king, or Chogyal, of Sikkim established his kingdom here in 1642, eventually ruling over the Lepcha and Limbu minorities already settled in this region. The royal family that moved to Sikkim had fled eastern Tibetan (said to be scions of the Minyak household) in the middle of the 13th century, no doubt due to trouble with the Mongol invasions of this region. They were called the Namgyel kings, and helped with the construction of Sakya Monastery in 1268. Having married into the ruling Sakya family, they were established as the rulers of the Chumbi valley, which became the basis for the later Sikkimese kingdom. The region of Sikkim was first described as "the best of all the sacred places of pilgrimage, as it will come to be resorted to n the end of the evil times..." by the 13th century treasure discoverer Ratna Lingpa. In the 14th century, another treasure discover named Gokyi Demtru chen transmitted a text about this hidden land (beyul). It was not until the predicted "evil times" of the mid-seventeenth century civil wars in Tibet that the family moved further south into the lower elevations to establish a new seat of power in Sikkim proper.

Source: Brigitte Steinmann, "The Opening of the sBas Yul 'Bras mo'i gshong according to the Chronicle of the Rulers of Sikkim: Pilgrimage as a Metaphorical Model of the Submission of Foreign Populations." In Pilgrimage in Tibet. Alex McKay, ed. London: Curzon Press. 1998. Pp. 117-143.