You'll be able to play the new Archenemy game type that Wizards of the Coast released back in the spring. It lets one player take on multiple opponents with an additional 20 overpowered cards in his deck. There will be an 11-level single-player campaign for the Archenemy mode.
Three new decks, six single-player campaign levels, four new cards for each of the existing decks, more achievements, and new loading screens are also going to be added as well. A preview of six cards from a later expansion called "Inistrad" is also included.
The expansion will be available for PC, PlayStation 3, and Xbox 360 this fall. If you're at PAX Prime you'll be able to play it at the Magic booth.