Toxrill, the Corrosive is a mythic Legendary Creature — Slug Horror card from Secret Lair Drop (SLD), collector number 1929. Current market price: $20.32 (foil $56.99), TCGplayer market prices via Scryfall.
At the beginning of each end step, put a slime counter on each creature you don't control. Creatures you don't control get -1/-1 for each slime counter on them. Whenever a creature you don't control with a slime counter on it dies, create a 1/1 black Slug creature token. {U}{B}, Sacrifice a Slug: Draw a card.