Shadow Slice (GTC) from Gatecrash, available at 401 Games Canada.

• Rarity: Common
• Finish: Normal
• Artist: Raymond Swanland
• Card Text:
  Target opponent loses 3 life.
  Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)