Cheapest priced copy (Urza's Legacy): $0.49 at Card Kingdom, near mint, their price list as of .
Opal Avenger 
Enchantment · Rare · Urza's Legacy · #15
When you have 10 or less life, if this permanent is an enchantment, it becomes a 3/5 Soldier creature.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.49 to $11.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Legacy | 15 | Rare | $0.49 | $11.99 |
2 rulings from Wizards of the Coast
- 4 October 2004When it turns into a creature, it is no longer an enchantment.
- 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.