Cheapest priced copy (Ikoria: Lair of Behemoths): $0.35 at Card Kingdom, near mint, their price list as of .
Cavern Whisperer 
Creature — Nightmare · 4/4 · Common · Ikoria: Lair of Behemoths · #79
Mutate {3}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Menace (This creature can't be blocked except by two or more creatures.)
Whenever this creature mutates, each opponent discards a card.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ikoria: Lair of Behemoths | 79 | Common | $0.35 | $0.49 |
| Ikoria: Lair of Behemoths | 287 | Common | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 17 April 2020As the triggered ability resolves, first the next opponent in turn order (or, if it’s an opponent’s turn, the opponent whose turn it is) chooses a card in hand without revealing it, then each other opponent in turn order does the same. Then all the chosen cards are discarded at the same time.