What if Zabergan Captures Constantinople?
Background
"The Kotrigurs soon resumed their raids, however, and this time with vengeance. Their war chief, who bore the Iranian name Zabergan, began a vast new expedition against the Eastern Roman Empire in late 558 AD. He boldly divided his host into three bands, each with a definite sphere of operations: one assigned to Greece, one to Western Thrace and the third, under his personal command, to eastern Thrace against Constantinople itself.
His audacity was justified by circumstances of the Balkan provinces at this twilight period. The emperor's military resources had been squandered or exhausted in his wars of reconquest. His treasury had been drained by decades of expenditures. His fortifications were currently being used mainly as places of refuge alone, and the dilapidated Long Wall, which was meant to protect the hinterland of Constantinople was of little value. The Kotrigurs ravaged at will bringing fire and sword to the population of Greece and Thrace. Worse of all they were soon pasturing their nomad horses in the suburban gardens of the capital.
Flavius Belisarius had been living in quiet retirement in Constantinople since his return from Italy. Although the old general had long ceased active service he had lost none of his robust energy. Quickly rallying to his emperor, he gathered what troops were available in the city. Most of them were worthless, though he had a core of some 300 veterans. With these pitifully limited forces, Belisarius marched out to defend the capital and what was left of Thrace. Through a masterly use of ruses and skillful positioning he tricked Zabergan's forces into thinking they were about to fall prey to a great army. They abandoned their attack on Constantinople and withdrew from Thrace."
Attila the Hun by Nic Fields
Zabergan doesn't fall for Belisarius' tricks and continues to besiege Constantinople until it falls. What are the results? Could Zabergan have absorbed Thrace and Greece into his empire? What would the impact have been on the Byzantine Empire?