Tengriism was brought to Eastern Europe by nomadic tribes migrating or invading from the central Asian steppes. The faith was very closely connected to the nomadic lifestyle, so that in most cases people changed their religion after turning sedentary.
The Danube Bulgars apparently called the sky god Tangra. They named a large mountain in the Rila mountain range of Bulgaria after him, only in the 15th century it was renamed to Musala (Mountain of Allah) by the Ottomans.
There are few occurrences of the name in documents related to Bulgaria. One is in a late Turkish manuscript listing the names of the supreme god in different languages, which has "Tangra" for Bulgarian. The other is in a severely damaged Greek language inscription from the times of Danube Bulgarian paganism. It is found on a column near Madara, Bulgaria, which is believed to have been used as an altar stone. The inscription has been interpreted as saying "(Kanasubig)i Omu(rtag), ruler (from God), was ... and sacri(ficed to go)d Tangra ...(some Bulgar titles follow)." In addition, Bulgarian historian Veselin Beshevliev has conjectured that the frequent Danube Bulgar runic sign ıYı stands for "Tangra", as it seems to disappear after the conversion to Christianity.
Tengriism (Tengerism, Tengrianism, Tengrianizm, Tengricilik) was the major belief of the Mongols and Turkic peoples before the vast majority joined the established world religions. It focuses around the sky deity Tengri (also Tangri, Tangra, Tanrı, etc.) and incorporates elements of shamanism, animism, totemism and ancestor worship.
"Tenger" literally means "sky" in Mongolian language and modern Mongolians still pray to "Munkh khukh tenger" ("eternal blue sky").