Medieval Slavs were named after the regions they settled.
Polochans settled on Polota river. Buzhane (Volynians) on Bug river. Polians in medows. Drevlyans in the woods. Moravians on Morava river. Dregovicihes on swamps . Dryhva (Dregva, Driagva) is a swamp in Belarusian and Ukrainian. Draguviti may have a different etymology.
East Slavic tribe Severians has unclear etymology. In the past eastern Slavic dialects used midnight for north. Midnight is still used for north in Belarusian, Ukrainian, Polish languages. Russians also used midnight and midday for north and south in the past.
Some scholars suggested that Severian name (their settlement was present day north-eastern Ukraine and and neighbouring Russia) has Iranic origins meaning black, as the region is full of Iranic hydronyms . The main city of Severians was Chernigov (Chernihiv).
Early scholars who supported the hypothesis of Dunube rigion being the birthplace of Slavs suggested Severians of Bulgaria migrated in the north to north-eastern Ukraine. This is unlikely in the light of recent research. The migration was in the opposite direction ie from north to south.