For these boosters, Mirrodin Besieged was divided into two halves. Cards with the Mirran watermarks appear only in Mirran boosters, and cards with Phyrexian watermarks appear only in Phyrexian boosters. There's also one basic land in each booster. Foil cards will show up at about the same frequency as they do in regular boosters and don't follow the rules about what booster they can show up in (both Mirran and Phyrexian foils can show up in either booster).
This is something that's happening only at the Mirrodin Besieged Prerelease Event. Players will receive three regular Scars of Mirrodin booster packs and their choice of three Mirran boosters or three Phyrexian boosters. This is something we’re doing especially for the Mirrodin Besieged prerelease; regular Mirrodin Besieged boosters will be, well, regular.
There will be two different prerelease promo cards provided for the prerelease. Everyone will identify as either Mirran or Phyrexian and should receive the corresponding card for that faction. Players who participate in multiple events are allowed to change factions between events.