Rulak (ru lag) is one of the two “ru” that make up Tsang (gtsang), which is the Western part of central Tibet (Ütsang). “Ru” means “horn” or “region.” Central Tibet is divided into four horns with two in Tsang and two in Ü (dbus). In a note in the Blue Annals, Roerich mistakenly explains that Rulak is the eastern region of Tsang, when it is the western region (Roerich, 205 note 8; Davidson 4). Lhatsè (lha rtse) and Sakya (sa skya) are a part of this region.
Rulak only appears twice in the Blue Annals. It is first mentioned in Chapter 3, "The Early Translations of the Secret Mantra," and we learn that Rongzom Chökyi Zangpo (rong zom chos kyi bzang po) was born in the region (Roerich 160). The only other appearance of a reference to Rulak in the Blue Annals is in Chapter 4, "The New Secret Mantra Traditions including the Path and its Results (lam ‘bras) Tradition." We learn that it was the three sons of the ruler of Rulak, Tri Trashi Tsekpa-pel (khri bkra shis brtsegs pa dpal, ca. 911-50), who requested that Lotön Dorjé Wangchuk (lo ston rdorje dbang phyug) send an upādhyāya and an ācārya to their region to propagate the Dharma. Lotön sent Shākya Zhönu (gzhon nu) and Sé Yéshe Tsöndrü, who ordinated Drokmi and sent him to India (Roerich, 205; Davidson 165). Davidson explains that the three sons, Peldé (dpal lde), Ödé ('od lde) and Kyidé (skyid lde), "were instrumental in the reintroduction of Buddhist learning into some areas in Central Tibet" (Davidson, 69).
Sources
The Blue Annals
TBRC
Davidson, Tibetan Renaissance