Cheapest priced copy (Urza's Legacy): $0.35 at Card Kingdom, near mint, their price list as of .
Opal Champion 
Enchantment · Common · Urza's Legacy · #16
When an opponent casts a creature spell, if this permanent is an enchantment, it becomes a 3/3 Knight creature with first strike.
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
1 printing · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Legacy | 16 | Common | $0.35 | $0.79 |
5 rulings from Wizards of the Coast
- 4 October 2004It triggers when the spell is cast, so it becomes a creature before the spell resolves. A creature that would destroy an enchantment when it enters can't be used to destroy this card, since it will no longer be an enchantment when the creature enters.
- 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.
- 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.