Mirrodin Beseieged, the small expansion for the Scars of Mirrodin block released on Feburary 4th 2011, and continues the story from Scars as the invasion of the Phyrexians destroys the rural areas of Mirra corrupting the inhabitants and shaping themselves into twisted mirrors of their enemy.
In recognition of the influence of Phyrexia, this set featured many Mirrored Pairs (different cards that shared an opposing effect) such as Peace Strider and Pierce Strider. This set also added Living Weapons, which allowed Equipment to come into play attached to a 0/0 creature, and Battle Cry.
Cards to look out for in this set include Tezzeret - Agent of Bolas, Mirran Crusader, Magnet Mine, Massacre Wurm, Bonehoard, Psychosis Crawler, Go for the Throat, and the utterly terrifying Blightsteel Colossus.
36 packs per box, each pack contains 16-Cards, 1 Rare/Mythic Rare (Mythic rares are approx. 1 in every 8 rares), 3 Uncommons, 10 Commons, 1 basic land, 1 Token/Tips & Tricks card.