 In the late 7th century the Seven tribes were assigned the defence of the newly established Bulgar Khanate's western and northwestern border (the Iskar River up to its mouth in the Danube) against Avar raids, as well as some of the passes of the Balkan Mountains, whereas the Severi, whose possible participation in the union is not clear, would guard the eastern part of the mountains.The Seven Slavic tribes, together with other Slavic tribes of the Bulgarian Empire, gradually formed the Bulgarian ethnicity in the 9th century due to the Christianization of Bulgaria under Boris I and the preceding administrative reforms that deprived them of their autonomy and self-government.The Asparuch Bulgars were not numerous and settled in the Balkans with a single migration wave, which Michael the Syrian described as numbering 10,000.