Has anyone else had a problem playing against a skaven player equipped with this card? From my and my friends' understanding the under empire allows a skaven player to chain destroy multiple territories fairly easily in descending order down the old world in one ruination phase (destroy one, move your army down to the next territory before it's 12 corruption is counted, repeat repeat repeat). True other players can step up to cull the herd so to speak but if they spread out their guys to fight the skaven hordes they have less warriors to defend their own territory so essentially one player has to choose to risk ruining their chance to win to stop the skaven from winning.

It seems like an incredibly broken upgrade. Maybe I'm missing something?

It's situational and not as powerful as you think.

You can only move figures from a region that will be ruined, which you cannot do on your own. Then you can only move figures from this ruined region to oneother region, sou you can't spread around. The Scenario you describe can happen, but only if 2 or more regions are ruined in the same round, and only if they all contain Skaven tokens.

And by moving all your figures out you're losing the points from 1st or 2nd ruiner. 

That is, you score the points for initial Ruiners (usually 3-5 pts) which all contributing ruiners that round score. But to "win" and get the big point payout you need to leave more figures in the region than any other power has CPs there. Read the wording on the cards, and the sequence in the rules:

Corruption & Domiation phase - You check for ruination, and Ruiners score points. At this point, Skaven player may move any number of figures out of the region.

The End phase - You score ruined regions. If the skaven figures have been moved away, they score no points.