Phyrexian Seedling

Proliferate normally lives in your upkeep or in an activated ability: you spend a card or a mana slot to add one more of everything you already have. This creature moves the mechanic into combat, and that relocation is the whole design idea. It arrives as a 1/1 (a 0/0 with a single +1/+1 counter, so it dies to anything that removes counters), and every point of combat damage it deals causes you to proliferate that many times before lethal-damage checks resolve. That ordering is the load-bearing wrinkle, and it works precisely because proliferate here is a static effect that modifies how the creature's damage is dealt, not a triggered ability waiting on the stack: like lifelink, it takes effect as part of dealing damage rather than after it. So the counters the Seedling gains from connecting can grow it past incoming blockers' damage inside the same combat step, and a favorable-looking block can turn into a dead blocker and a survivor. Because proliferate touches everything with counters, a single unblocked hit compounds across planeswalker loyalty, saga chapters, poison, energy, and every other counter you have in play. The body starts small on purpose; the payoff scales with how much of your board is already counting something. It converts the act of attacking, rather than the act of paying mana, into the proliferate engine, and folding that inversion into a damage-replacement effect is the trick that no earlier proliferate card pulled off.