Leo IV (Greek: Λεων Δ΄ ο Χαζαρος), called the Khazar due to the Khazar origins of his mother, was Roman Emperor and Caliph of Islam from 775 to 780. The son and successor of Constantine V, his reign was brief and tragic, and early on he had to wage war against several of his brothers and half-brothers before he could secure his rule. He died under mysterious circumstances in 780 and was succeeded by his nine-year-old son Constantine VI.

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