Cheapest priced copy (Khans of Tarkir): $0.99 at Card Kingdom, near mint, their price list as of .
Pearl Lake Ancient 
Creature — Leviathan · 6/7 · Mythic · Khans of Tarkir · #49
Flash
This spell can't be countered.
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Return three lands you control to their owner's hand: Return this creature to its owner's hand.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Khans of Tarkir | 49 | Mythic | $0.99 | $0.99 |
4 rulings from Wizards of the Coast
- 20 September 2014Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
- 20 September 2014Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.
- 20 September 2014A spell or ability that counters spells can still target Pearl Lake Ancient. When one resolves, Pearl Lake Ancient won’t be countered, but any additional effects of that spell or ability will still happen.
- 20 September 2014Any spell you cast that doesn’t have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won’t cause prowess to trigger. Playing a land also won’t cause prowess to trigger.