Kievan Rus'
"Tsar" was used once by Church officials of Kievan Rus' in the naming of Yaroslav the Wise of Kiev. This may be connected to Yaroslav's war against Byzantium and to his efforts to distance himself from Constantinople. However, other princes of Kievan Rus' never styled themselves as "czars". After the fall of Constantinople to the Crusaders and the Mongol invasion of Rus' (1237–1240), the term "tsar" was applied by some people of Kievan Rus' to the Mongol (Tatar) overlords of the Rus' principalities.
Read more about this topic: Tsarisem