This is a Mirrodin Besieged Faction Booster Box. This box was only given out to Authorized WPN Retail stores that ran Prerelease Tournaments. These boxes are extremely rare, and they will never be released again.
This box still contains 36 booster packs like a normal box, but the big difference is that there are 18 Mirran Packs, and 18 Phyrexia Packs. The packs ONLY contain cards from the specified faction.