Cheapest priced copy (Time Spiral): $0.35 at Card Kingdom, near mint, their price list as of .
Slipstream Serpent 
Creature — Serpent · 6/6 · Common · Time Spiral Remastered · #86
This creature can't attack unless defending player controls an Island.
When you control no Islands, sacrifice this creature.
Morph {5}{U} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Time Spiral | 77 | Common | $0.35 | $0.49 |
| Time Spiral Remastered | 86 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 19 March 2021Slipstream Serpent's first ability applies only while you're declaring attackers. If it attacks while face down and then is turned face up, it's still attacking even if the defending player controls no Islands.
- 19 March 2021If a creature attacks a planeswalker, that planeswalker's controller is the defending player.