This one addresses one and only one balance issue: shadow bats outside of The Deep, particularly on harder difficulties. These guys really could make the overlord's keep unwinnable in certain circumstances.
This is a standard update that you can download through the in-game updater itself, if you already have any version of the game. If you have the beta on Steam, it will automatically update for you. When you launch the game, you'll see the notice of the update having been found if you're connected to the Internet at the time. If you don't have the standalone game, you can download that here. If you already own the first game, just use your existing license key to unlock the sequel for free!