 Complete the following phrase:  By the 880s, Khazar control of the Middle Dnieper from Kyiv, where they collected tribute from Eastern Slavic tribes, began to wane as Oleg of Novgorod wrested control of the city from the Varangian warlords Askold and Dir, and embarked on what was to prove to be the foundation of a Rus empire. The Khazars had initially allowed the Rus to use the trade route along the Volga River, and raid southwards. See 