Opal Titan 
Enchantment
When an opponent casts a creature spell, if this permanent is an enchantment, it becomes a 4/4 Giant creature with protection from each of that spell's colors.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL