Cheapest priced copy (Urza's Saga): $0.49 at Card Kingdom, near mint, their price list as of .
Opal Titan 
Enchantment · Rare · Urza's Saga · #26
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.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Saga | 26 | Rare | $0.49 | — |
6 rulings from Wizards of the Coast
- 4 October 2004It changes into a creature even if the spell is countered.
- 4 October 2004When it turns into a creature, it is no longer an enchantment.
- 4 October 2004If the spell was colorless (an artifact), this card does not get any form of Protection.
- 4 October 2004It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
- 1 April 2008A "creature spell" is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.
- 1 August 2008A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.