Sludge Monster is a rare Creature — Horror card from Innistrad: Midnight Hunt Promos (PMID), collector number 76p. Current market price: $0.96 (foil $0.65), TCGplayer market prices via Scryfall.
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.