Cheapest priced copy (Innistrad: Double Feature): $0.79 at Card Kingdom, near mint, their price list as of .
Grolnok, the Omnivore 
Legendary Creature — Frog · 3/3 · Rare · Innistrad: Crimson Vow · #324
Whenever a Frog you control attacks, mill three cards.
Whenever a permanent card is put into your graveyard from your library, exile it with a croak counter on it.
You may play lands and cast spells from among cards you own in exile with croak counters on them.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.79 to $7.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Innistrad: Double Feature | 505 | Rare | $0.79 | $7.99 |
| Innistrad: Crimson Vow Promos | 238s | Rare | — | $5.49 |
| Innistrad: Crimson Vow | 238 | Rare | $0.99 | $0.79 |
| Innistrad: Crimson Vow | 324 | Rare | $0.79 | $1.49 |
2 rulings from Wizards of the Coast
- 19 November 2021Grolnok's last ability allows you to play lands and cast spells from among all cards you own in exile with croak counters on them, regardless of whether they were put there by the Grolnok that's currently on the battlefield or by an earlier incarnation.
- 19 November 2021Grolnok, the Omnivore's second ability exiles all permanent cards that are put into your graveyard from your library, not just those that you mill when a Frog you control attacks.