The peak of the Slavic expansion in Greece is showed by this map:
Slavic tribes in mainland Greece and in the Peloponnese - among them
Milingowie (Μιληγγοί; Milengoi) and Jeziercy (Ἐζερῖται; Ezerītai):
About Slavic Milingowie / Milengoi (google translated to English): http://translate.google.com/translate?sl=pl&tl=en&u=http%3A%2F%2Fpl.wikipedia.org%2Fwiki%2FMilingowie
About Slavic Jeziercy / Ezeritai (google translated to English): http://translate.google.com/translate?sl=pl&tl=en&u=http%3A%2F%2Fpl.wikipedia.org%2Fwiki%2FJeziercy
Map:
http://s21.postimg.org/sv442v3qv/Slavs_in_Greece.png
As you can see western part of the Peloponnese Peninsula was known as "Sklavonia terra" since the 8th century (VIII w.).
Jeziercy and Milingowie during the 10th century (X w.) lived even further south than Slavs of "Sklavonia terra".
And let's add that this map is from Zofia Kurnatowska's Polish book "Słowiańszczyzna południowa" ("Southern Slavdom").
Below I marked approximate boundaries between territories of different Slavic tribes from this map with red colour:
After that, I tried to guess who lived in other territories (i.e. those where no Slavic tribes were present) - blue names:
BTW - in the northern part of the map, words cut in half: "... VACI" = Croats and "... RBOWIE" = Serbs:
http://postimg.org/image/wiqcli8n3/
Well - there should be also Aromanians somewhere there (perhaps mixed with others in the same territories).
I forgot about them.
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As for Slavic tribes of Milengoi and Ezeritai in in the Peloponnese - they remained Slavic-speaking until at least year 1200.
Only after that they became Greek.... what is the word ??? You know, the verb created from noun "Greekization".
They underwent Greekization after ca. year 1200.
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BTW:
Within the Byzantine Empire, Milengoi and Ezeritai enjoyed large
autonomy - their tribal territories existed as separate administrative
districts.
Byzantine Emperors also allowed the Milengoi and the Ezeritai to have
their own Dukes / Princes, who ruled their respective autonomous
regions.
Considering how many privileges they enjoyed, I wonder if they came to
the Peloponnese on their own, or were invited there by Emperor.
This question arises especially when we see, that their area was
separated from other Slavic-speaking areas by non-Slavic regions.
And here another part of this map by Kurnatowska - areas further to the North:
Violet colour = Langobards and Gepids - who were there only until 567 /
568 (Langobards escaped from there to Italy due to the Avar invasion):
Orange colour = Southern Slavs (Sclaveni) in early 500s - later they
abandoned that area and invaded from that area into the Balkans:
Green colour = steppe people (Avars - since 568 and Proto-Bulgars - since the 7th century):
http://postimg.org/image/mh0yx0ua5/
Karantanie = Carantanians (ancestors of modern Slovenes)
Słowianie Morawscy = Moravian Slavs
Chorwaci = Croats
Serbowie = Serbs
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Here a "clear" version:
http://postimg.org/image/qrm5n7zb5/