I just got my copy of PCGamer in the mail today where they gave a paragraph or so to CA's showing at E3.
From PCGamer(US), pg.40:
"Explains Development Director Mike Simpson: ""We're talking about a period where thte Western Roman Empire was overrun by successive waves of different barbarians-Goths, Visigoths, Vandals, Franks, etc.-Which were in turn being displaced by a huge invasion of Horse Tribes, the Huns, from the east.""
It goes on to say that the exp pack will add new units and tech trees for all the factions, religions, night battles, new traits and characters, new historical events, and a reskinned interface for the Barbarians.
Check out PCGamer for the screenshots, etc...
-edit- sorry if this is a redundant post, I just saw the one about Barb. Inv. posted a couple hours ago.