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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.

En résumé : à l'AP on aura 3 boosters SoM normaux, et 3 boosters MBS alignés, de la même faction choisi par le joueur. Il y aura également 2 cartes d'AP différentes (logiquement, on obtiendra celle affiliée à la même faction que les boosters choisis) donc pour avoir les deux, il faudra faire 2 AP ;)

source : Arcana