It's a celebration! People are dancing in the streets, and riding horses through the dancehalls. You've finally formed an alliance with the barbarians to the north. Instead of the streets running red with blood, they'll run, well, the usual color, let's not focus on what color the streets run. The point is, there's peace. Sure negotiations were tricky. The barbarians are uncouth; they have no five-second rule and stick out the wrong finger when drinking tea. There are perks too though. They've given you skulls to drink mead out of, and spices to get rid of the skull aftertaste. And you've given them stuff in return: forks, mirrors, pants. It's great for everyone. And with this treaty out of the way, you can get to work on your other neighbors. Soon, all the allies will be yours.
Allies is the 14th expansion to Dominion. It has 400 cards, with 31 new Kingdom card piles. There are Allies that will do Favors for you, and split piles you can rotate.