The Empire of the Avars was an empire that existed from the 6th to the 8th century, so at a time, where heraldry and flags have not played in a major role. The flag of the Avars is probably a legend and has in this way probably never existed.
The Avars have been a nomadic Turkic people with controversial origins. They were ousted by the nomadic Turks from western Asia and from the end of the 6th century they threatened the Byzantine Empire. After the withdrawal of the Lombards to Italy, the Avars occupied the territory of today's Hungary and established an empire from the Elbe River to the Black Sea, and from the Dniester River (Tyras) to the Danube River. Between 791 and 803 the Frankish Empire under Charlemagne fought several campaigns against the Avars, the Empire of the Avars perished because of this, and the Avars were fully assimilated by the Slavs and by the Hungarians.
