Cheapest priced copy (Commander Legends: Battle for Baldur's Gate): $0.79 at Card Kingdom, near mint, their price list as of .
Sludge Monster 
Creature — Horror · 5/5 · Rare · Innistrad: Midnight Hunt · #76
Whenever this creature enters or attacks, put a slime counter on up to one other target creature.
Non-Horror creatures with slime counters on them lose all abilities and have base power and toughness 2/2.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $0.79 to $10.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends: Battle for Baldur's Gate | 735 | Rare | $0.79 | — |
| Innistrad: Double Feature | 76 | Rare | $1.49 | $10.99 |
| Innistrad: Midnight Hunt | 76 | Rare | $0.99 | $0.99 |
| Innistrad: Midnight Hunt | 340 | Rare | $0.79 | $1.49 |
| Innistrad: Midnight Hunt Promos | 76p | Rare | $0.99 | $1.99 |
| Innistrad: Midnight Hunt Promos | 76s | Rare | — | $2.99 |
2 rulings from Wizards of the Coast
- 24 September 2021If a non-Horror creature with a slime counter on it somehow becomes a Horror, the effect making it 2/2 and making it lose its abilities ends. Its base power and toughness will revert to whatever it was before that effect started applying, although other effects that modify power and/or toughness, such as ones created by Auras, counters, and the like may still apply. It'll also have whatever abilities it had before it lost them.
- 24 September 2021Sludge Monster's last ability affects all non-Horror creatures with slime counters on them, not just creatures that had slime counters put on them with Sludge Monster's first ability.