The Tractatus de ortu Tartarorum ("Treatise on the Rise of the Tartars") is a Latin treatise on the Mongols (Tartars), consisting of answers given by a Russian bishop named Peter to questions posed by Pope Innocent IV and the College of Cardinals in late 1244. The Tractatus originally circulated among the clergy assembled for the First Council of Lyon in 1245. It had a profound effect on the pope, convincing him to send embassies to the Mongols to negotiate peace.
The purpose of Peter's mission to the West and who, if anybody, was behind it have provoked much speculation. He has been seen as a refugee from the Mongol invasion acting on his own initiative, as an envoy sent by the Mongols to sow disinformation and as a representative of the Russian church seeking to advance the cause of church union. At the time of his visit, he was the acting vicar of the metropolitanate of Kiev and the effective leader of the Russian church. He participated in the Council of Lyon.
The Tractatus contains a lengthy section on Mongol origins that is based mainly on an interpretation of the Apocalypse of Pseudo-Methodius. The rest of the work contains original information, seemingly supplied by a son-in-law of Genghis Khan, either an exile or a spy. It is mostly an accurate account, albeit one that magnifies both the Mongols' strength and their willingness to negotiate.