Carnifex Demon

The two -1/-1 counters it walks in with are the whole engine, not a drawback. Read the body as a 6/6 flyer and the entry counters look like a tax; read them as a fuel tank and they become two charges of a symmetric board sweep priced at a single black mana each. Every activation removes one of those counters to drop a -1/-1 on every other creature, which means the Demon itself grows back toward its printed 6/6 as the board around it shrinks: pay , peel a counter off the creature, and the rest of the battlefield (yours included, since the effect hits each other creature) gets smaller while it gets bigger. That symmetry is the real wrinkle. The Demon is the only thing exempt, so the trick is fielding nothing else worth losing, or building your own board out of things that profit from dying. The math punishes go-wide hardest: a field of X/1 tokens evaporates on the first activation. The structural cleverness is that the counters live on the creature rather than on a global clock, so the ability is self-limiting by design (two shots unless something feeds it) and openly abusable by anything that can proliferate them. Strip the two it came in with and you have a clean 6/6 flyer; proliferate them and you have a repeatable sweeper bolted to a finisher. The resource powering the effect is the same resource defining the body, and crucially they move in the same direction.