Nemesis Mask (DST) from Darksteel, available at 401 Games Canada.

• Rarity: Uncommon
• Finish: Normal
• Artist: Adam Rex
• Card Text:
  All creatures able to block equipped creature do so.
  Equip 3 (3: Attach to target creature you control. Equip only as a sorcery. This card comes into play unattached and stays in play if the creature leaves play.)