if only the classical writers of the ancient world had bothered to learn about the actual Proto-Slavs and Proto-Balts and Bolgars, Proto-Turks of Eastern Europe, we could actually know something rather than guessing and leaving it at "they spontaneously generated in the Dark Ages".

Another problem is that even if they existed they were not organized enough to be a faction, although that is also a reason for their veiled existence, because the open-natured membership of the Goths and Huns meant that if you lived similar and served them, you were them. so the dominant Steppe peoples such as the Antes and Avars for a long time made their identity seemingly unimportant.

the West Slavs especially seem to exist very early but are mixed up by everybody as Vandals or just cannot be "proven" to be the Lugi or Wends... yet all of a sudden in the Dark Ages, you have Sorbs, Obodrites, Pomeranians all very deep into Central Europe... it's interesting that Bohemia also then becomes mixed even more than it was to begin with that places is really an awesome hub.

Great Moravia was such an interesting and significant early Eastern European culture / civilization though, it's too bad we have nothing comparable in an earlier timeline.