One of these people wrote the Schechter Letter. One ruler of these people led them through theDarial Pass into Azerbaijan where he then won a major victory at the Battle of Ardabil; that leaderwas named Barjik. After their kagan Bulan made a religious conversion, they started issuing coinswith the message that Moses was God's true prophet. Byzantine emperors Constantine V 
One of these people wrote the Schechter Letter. One ruler of these people led them through theDarial Pass into Azerbaijan where he then won a major victory at the Battle of Ardabil; that leaderwas named Barjik. After their kagan Bulan made a religious conversion, they started issuing coinswith the message that Moses was God's true prophet. Byzantine emperors Constantine V andJustinian II (*) married these people, and Leo IV's epithet indicated he was one of these people. Thepower of these people declined after Syvatoslav I of Kiev conquered their stronghold of Sarkel and capitalof Itil in the 10th century. For 10 points, name these Turkic-speaking people of Eurasia whose eliteconverted to Judaism.
One of these people wrote the Schechter Letter. One ruler of these people led them through theDarial Pass into Azerbaijan where he then won a major victory at the Battle of Ardabil; that leaderwas named Barjik.
Khazars [or Kuzarim or Hazarlar or Xazarlar of Gazan or Cosn or Gasani]