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, creature a 1/1 black slug creature token.
ub, sacrifice a slug: draw a card.
Toxrill, the Corrosive from Promo Pack: Dominaria United