IGN reports: "Marvel: Ultimate Heroes and Villains expansion adds eight new playable characters. Joining the aforementioned green behemoth are Cyclops, Nightcrawler, Hawkeye, Magneto, Venom, Sabretooth and Doctor Doom. Each character comes with new signature powers, skins, dialogue and conversations. And yes, 10 new Achievements are being added, bringing Ultimate Alliance's total to a whopping 60 possible Achievements."
Apparently there will be one pack of four "Heroes" and one pack of four "Villians" which will sell for 500 Microsoft points each or you can buy a single pack of all 8 characters for 800 Microsoft points.