Swarm of Bloodflies

A 0/0 that walks in as a 2/2 flier, and that gap between the printed body and the working body is the whole design. The two entry counters do the structural work an x/x base stat would do anywhere else, but routing the toughness through counters means the creature has room to grow: every other creature that dies, on either side of the table, feeds it. That makes it a death-payoff that does not care who is doing the dying. A chump-block into your attacker, a trade in combat, an opponent's creature you kill with removal, your own sacrifice fodder: each one banks another counter, and the flying converts those accumulated stats into damage rather than stranding them behind a ground wall. The catch is the same fragility that makes counters cheap. The base body is 0/0, so anything that subtracts toughness faster than the counters accrue kills it outright: a -2/-2 erases a fresh copy before a single creature has died. A symmetric board wipe is worse than it looks, because the Bloodflies dies to the wipe itself and leaves the battlefield alongside the other casualties, so even though its triggers go on the stack the counters have nowhere to land, and it eats none of them. Bounce is the cleanest answer of all, sending it home and discarding every banked counter, so the rebuilt copy starts over as a bare 2/2. It is a snowball wearing flight, threatening only once bodies are already dying in numbers, which makes the grindy games the ones where it matters.