Where has the "Eternal Blue Sky" or "Tengri" been worshipped?

Tengrism, or also known as Tengriism, Tengerism, or Tengrianism, is a ancient central Asian shamanistic religion that centered around the sky god Tengri. Ethnic groups, including Bulgars, Turkic tribes, Mongol tribes and others in the central Asian steppes worshipped Tengri as the "God of Heaven''. In some cases, it was considered a montheistic religion (one God), others included Tengri as one among other Gods, or as a shamanistic deity. Shamanism viewed articles found in nature (the sky, earth, trees, etc) as embodiments of gods or spirits. Genghis Khan was considered a vessel or embodiment of Tengri among the Mongol tribes. Mongolia is sometimes termed as the "Land of the Eternal Blue Sky"

The empire founded by Genghis Khan (a several of the successor states) kept Tengriism as the "state" religion, although other religions were accepted within the Mongol borders as well. Mongol khans were considered "Shaman-Kings" for several generations, until Genghis Khan's 5th-generation successor, Uzbeg Khan embraced Islam.