The new expansion is entitled "Rise of the Horned Rat." In addition to many other changes and additions, skaven are being added as third a playable faction. If you are any kind of fan of the Warhammer miniatures game, then you were likely disappointed that skaven weren't in WAR originally (I certainly was). The fact that they are being added at all has me pretty excited.
The addition of a third faction is even better. When there are only two factions, there is no real system of checks and balances and one side tends to get into a hole they can never climb out of. You can see this very clearly in modern WAR, when on the majority of servers either Destruction or Order pretty much completely dominates end game RvR. In some cases, even lower tier PvP is affected, and new players of the underdog faction tend to leave or change sides after a few evenings of getting their butts handed to them relentlessly.
To me this expansion signals to me the Mythic finally "gets" that they made a some major mistakes in their original design. In my opinion, the third faction really should have been launch WAR.
Edit: indeed, as Jaydub points out, the expansion announcement is completely fake. Hit the clarification link at the top of the page I linked. Sorry to get your hopes up :-)