Justinian I, who assumed the throne of the Byzantine Empire in 527, oversaw a period of Byzantine expansion into former Roman territories, and re-absorbed the area of Kosovo into the empire. Slavic Invasions and migrations Slavic migrations to the Balkans took place between the 6th to 7th centuries. The region had been part of the Roman and the Byzantium until the first major Slav raids took place in the middle of Justinian's reign. In 547 and 548 the Slavs invaded the territory of modern Kosovo, and then got as far as Durrës on the Northern Albanian coast and reached all the way down to Greece. The overwhelming number of municipalities in modern-day Kosovo being Slavic in their toponymy suggests that these Slavic raiders either assimilated, or expelled the local non-Slavic populations inhabiting the region of Kosovo prior to their arrival. According to some historians, although some Slavs did spread out through these areas, there is one intriguing argument that suggests Slavic settlement in Kosovo and the Southern Morava Valley was weak in the first one or two centuries of Slavic settlement. According to some historians and linguists, if Slavs had spread evenly in this part of the Balkans it would be hard to explain the clear linguistic division that occurred between the Bulgarian-Macedonian and Serbo-Croat language. The scholar who first proposed this theory also noticed in the area that divided the early Serbs and the Bulgarians many Latin place-names in these areas survived long enough to be eventually adapted into Slavic ones, such as Naissus (Nis), Lypenion (Lipljan), Scupi (Skopje) etc. The plague of Justinian had killed millions of native Balkan people and as a result many regions had become depopulated and neglected by the government, this gave the Slavs a chance into raiding and settling in the Balkans. According to De Administrando Imperio, the ancestors of Serbs and Croats were part of the Slavic migrations into the Balkans, the Croats settled in modern Croatia and Western Bosnia whereas the Serbs in the rest of Bosnia, Travunija, Zahumlje and Duklja, lands situated North-West of Kosovo. The Serbs settled in the Rasca region. Middle Ages According to historian Noel Malcolm the Vlach-Romanian and Aromanian languages originated in the region and surrounding areas from Romanised Illyrians and Thracians, possibly a contact zone between the Albanian and Romanian language. Bulgarian Period The region of Kosovo was incorporated into the Bulgarian Empire during the