Sometime between 533–34 and 545 (probably before the 539–40 Hun invasion)

 https://en.wikipedia.org/wiki/Sclaveni

In 539 there was a major Hun attack on Greece and Thrace which even crossed the Dardanelles into Asia. 

 

 

 

Around the same time the Huns invaded the empire reaching Constantinople before retreating.^[1] A second invasion occurred shortly afterwards, bypassing Byzantine resistance and plundering all of Greece except the Peloponnese.^[1]

https://en.wikipedia.org/wiki/Siege_of_Auximus

 

 

 

Procopius tells us that the Huns had invaded the Empire frequently before this, but never had the calamity for the Romans been so great. In ad 539 they capture

 

https://www.cambridge.org/core/books/huns-rome-and-the-birth-of-europe/later-huns-and-the-birth-of-europe/