New race and not much else!
The expansion adds the dark elves, they are a good race with a whole load of nice units but the missions restrict you so you won't see much use of some of them. There are also about 7 or 8 neutral units you can also get if i am correct...things like cats and undead dragons.
There isn't many new things added to the game and i wouldn't mind that if they had bothered to add a lot more mission content but they didn't. The story is kind of poor as it all seems half finished and pointless. Its more like a big dlc than an expansion.
I would have liked a longer campaign possibly getting to play one with the faction of my choice. The campaign was pretty average, the story isn't really gripping and you will be left disappointed in the end.