Gary, the Snail - Toxrill, the Corrosive [Secret Lair Drop Series]

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.

UB, Sacrifice a Slug: Draw a card.