Seven Slavic tribes
The Seven Slavic tribes or the Seven Slavic clans (Bulgarian: Седемте славянски племена, Sedemte slavyanski plemena, or Bulgarian: Седемте рода, Sedemte roda) were a union of tribes in the Danubian Plain, that was established around the middle of the 7th century and took part in the formation of the First Bulgarian Empire together with the Bulgars in 680−681.
Since its establishment it faced attacks of the Byzantine Empire. The Bulgars crossed the Danube and in the 670's they have concluded an alliance with the local union. Theophanes writes that the Bulgars became masters of the Slavs. Slavic union recognized the sovereignty of Khan Asparukh and together, in the spring of 681, they won a major victory over the Byzantine troops.
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 Bulgars were less numerous and therefore gradually assimilated into Slavic culture, until by the 10th century they had adopted the Slavic language, with the Bulgar language going extinct in the kingdom.