An interdisciplinary study on the Khazar Khaganate and its place in the Eurasian space analyzes the data of Arab-Persian essays on the historical geography, history, genealogy of the Khazars. Fragments of some geographical works are published. New archaeological materials are attracted, showing the wide borrowing of building materials and technologies of Byzantine origin in the largest Khazar fortresses. A comparative historical analysis of the plots of the choice of faith in Khazaria and Russia demonstrates the reasons why the rulers who accepted the new faith could not abandon the pagan genealogical tradition: it was the basis of the legitimacy of their power.