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In the year 669 the Ugrians or Zabirs freed themselves from the rule of the Obrians, settled between the Don and the Caucasus, and came under the dominion of the Chazars. For this reason the Ugrians, who had hitherto been called the" White" or "Independent" Ugrians, are described in the chronicles ascribed to Nestor as the "Black," or "Dependent," Ugrians. They were no longer governed by their own princes, but were ruled by the kings of the Chazars. In 735, when the Arab leader Mervan moved from Georgia against the Chazars, he attacked the Ugrians also. In 679 the Chazars subjugated the Bulgars and extended their sway farther west between the Don and the Dnieper, as faras the head-waters of the Donetz in the province of Lebedia (K. Grot, "Moravia i Madyary," St. Petersburg, 1881; J. Danilevski and K. Grot, "O Puti Madyars Urala v Lebediyu," in "Izvyestiya Imperatorskavo Russkavo Georaficheskavo Obshchestva," xix.). It was probably about that time that the chaghan of the Chazars and his grandees, together with a large number of his heathen people, embraced the Jewish religion. According to A. Harkavy ("Meassef Niddaḥim," i.), the conversion took place in 620; according to others, in 740. King Joeph, in his letter to Ḥasdai ibn Shaprut (about 960), gives the following account of the conversion:
(see Harkavy, "Soobshchenija o Chazarakh," in "Yevreiskaya Biblioteka," vii. 153)
"Some centuries ago King Bulan reigned over the Chazars. To him God appeared in a dream and promised him might and glory. Encouraged by this dream, Bulan went by the road of Darlan to the country of Ardebil, where he gained great victories [over the Arabs]. The Byzantine emperor and the calif of the Ishmaelites sent to him envoys with presents, and sages to convert him to their respective religions. Bulan invited also wise men of Israel, and proceeded to examine them all. As each of the champions believed his religion to be the best, Bulan separately questioned the Mohammedans and the Christians as to which of the other two religions they considered the better. When both gave preference to that of the Jews, that king perceived that it must be the true religion. He therefore adopted it".
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