Crystalline Nautilus

Bestow {3}{U}{U} (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."