Crystalline Nautilus (JOU) from Journey into Nyx, available at 401 Games Canada.

• Rarity: Uncommon
• Finish: Normal
• Artist: Brad Rigney
• Card Text:
  Bestow 3UU (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
  When Crystalline Nautilus becomes the target of a spell or ability, sacrifice it.
  Enchanted creature gets +4/+4 and has "When this creature becomes the target of a spell or ability, sacrifice it.