As previously revealed, you will choose your faction for the Mirrodin Besieged prerelease (taking place on Jan. 29-30). With that faction choice, it defines not only the packs you receive but also which promo card you will receive.
We've already seen the cards, but today Wizards gave us better pictures of Glissa, the Traitor, Hero of Bladehold and Thopter Assembly.
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Source: Wizards.com