Oh, I see that article about Early Slavs (Wikipedia) was updated:
Jordanes wrote of the "Venethi", the "Sclavenes" and the "Antes", adding that all three ethnonyms referred to the one and same people (...)
Procopius completed his three works on Emperor Justinian I's reign.
Please see here: /wiki/Early_Slavs#Historiography
The location of Lechitic Radimichs and Vyatichs, who migrated from Central Europe to Ukraine and Russia before the year 885:
R1a-M458 (so-called West Slavic marker) distribution map:
It seems that Lechitic/Venethi Radimichs and Vyatichs preserved the rituals of the Iron Age ancestors (see Pomeranian, House Urns, Przeworsk, Oksywie and other cultures where use of burial urns was so popular):
Whenever a death occurred, a feast was held over the corpse, and then a great pyre was constructed, on which the deceased was laid and burned. After the bones were collected, they were placed in a small urn and set upon a post by the roadside, even as the Vyatichians do to this day.
Not only Radimichs and Vyatichs migrated from territories of Poland/East Germany...
Craniometric Relationships among Medieval Central European Populations: Implications for Croat Migration and Expansion, 2004
Of all 25 analyzed population samples, modern Croats exhibited greatest similarities with samples from the Ukraine, Hungary, and Poland (...) The results of these analyses are fairly consistent with the report written by the anonymous writer in the 30th chapter of the "De Administrando Imperio". Principal components analysis suggested that early medieval Croats were of Slavic origin and shared a common homeland with early medieval Poles, possibly in modern Poland and almost certainly in "... an area north of Bavaria ..." from where they migrated to the east Adriatic coast.
The positions of the analyzed Croatian sites in relation to the observed clusters were as follows. Sites from the east Adriatic coast: Nin, Bribir, Mravinci, and Danilo were located in the lower right part of the plot in the cluster of Polish sites. Nin, the most important early medieval Croatian site, occupied almost the same position as Cedynia, an early medieval site from northern Poland.
About arrival of Croats to Dalmatia during the reign of Heraclius (610-641):
The Croats who now live in the region of Dalmatia are descended from the unbaptized Croats, also called "white", who live (...) next to Francia, and have for Slav neighbours the unbaptized Serbs. "Croats" in the Slav tongue means "those who occupy much territory" (...) And so, by command of the emperor Heraclius these same Croats defeated and expelled the Avars from those parts, and by mandate of Heraclius the emperor they settled down in that same country of the Avars, where they now dwell.