Toxrill, the Corrosive

A board wipe that arrives one creature at a time. Most mono-black sweepers hit once and clear the table; this one grinds an opponent's board down over successive end steps, stacking slime counters that widen the -1/-1 penalty each turn until even fat threats melt. That slow-motion cadence is the whole design: play Toxrill and the opponent is on a clock where every creature they already have is dying and every new one is born into a shrinking window. The delay is the price of the effect. A seven-mana 7/7 that killed the opposing board immediately would be flatly unfair; letting them respond, sacrifice, or race the counters for a turn or two is the friction that makes a repeatable one-sided wrath printable at all. Two riders complete the engine. Each creature that dies with a slime counter feeds you a 1/1 Slug, so the sweep converts an opponent's board into your board, and those Slugs aren't chaff: the blue-black activated ability turns each into a card, giving the deck a grindy draw outlet that runs entirely off the wreckage. That splash of blue points squarely at multiplayer tables, where the counter accumulation snowballs across several opponents at once and the token math compounds. It is a control finisher dressed as a fatty: not a card that wins on the swing, but one that suffocates everyone else's board while quietly refilling your hand.