"King of the Khazars" refers to the ruler of the Khazar Empire, a significant medieval state that existed from the 7th to the 10th centuries in what is now southern Russia and parts of Ukraine. The Khazars were known for their unique position as a trade hub between Europe and Asia, facilitating commerce and cultural exchange. The Khazar Empire is particularly notable for its conversion to Judaism in the 8th or 9th century, making it one of the few states in history to adopt this religion as a ruling faith. The king's decision to embrace Judaism is often attributed to political motives, aiming to unify diverse groups within the empire.