Ong Khan
Ong Khan, alternately known as Unc Khan, Toghril, Tooril Khan, Unach, King David, and King John was the Khereid ruler in the late 12th/early 13th centuries. The title ong is the Mongol form of the Chinese honorific wang meaning 'universal' (Hamilton, p. 248).
Reportedly a Nestorian Christian– although according to Bar Hebraeus' Chronicon Syriacum, a lapsed one– "Unach" Khan was killed by his blood brother and subordinate, the soon-to-be Genghis Khan, who was acting on rumors that a jealous Unc was planning to kill him.
This is the same figure known in other mid-thirteenth century texts as King David, son of Prester John.
Ong Khan figures in a number of Prester John narratives, including the Historia Tartarum (c. 1246), William of Rubruck's Itinerarium (c. 1253), the Chronicon Syriacum, Marco Polo's Travels,