Cheapest priced copy (Urza's Saga): $0.35 at Card Kingdom, near mint, their price list as of .
Hidden Ancients 
Enchantment · Uncommon · Urza's Saga · #260
When an opponent casts an enchantment spell, if this permanent is an enchantment, it becomes a 5/5 Treefolk creature.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
1 printing · $0.35 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Urza's Saga | 260 | Uncommon | $0.35 | — |
3 rulings from Wizards of the Coast
- 4 October 2004When it turns into a creature, it is no longer an enchantment.
- 4 October 2004It changes into a creature even if the spell is countered.
- 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.