Cheapest priced copy (Dominaria Remastered): $0.35 at Card Kingdom, near mint, their price list as of .
Veiled Serpent 
Enchantment · Common · Urza's Saga · #110
When an opponent casts a spell, if this permanent is an enchantment, it becomes a 4/4 Serpent creature with "This creature can't attack unless defending player controls an Island."
Cycling {2} ({2}, Discard this card: Draw a card.)
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria Remastered | 72 | Common | $0.35 | $0.49 |
| Urza's Saga | 110 | Common | $0.35 | — |
4 rulings from Wizards of the Coast
- 4 October 2004It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
- 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 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.