Gary, the Snail - Toxrill, the Corrosive (1929) [Secret Lair Drop]

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.