Sprout Swarm

Two mechanics that should never share a card share this one, and the gap between them is where the engine lives. Buyback charges a steep tax to return the spell to your hand; Convoke lets your creatures pay for the spell by tapping. Cast it once to make a 1/1 green Saproling, and the loop announces itself: because Convoke lets a creature pay for one mana of its own color, and the token is green, five tapped Saprolings cover the entire cost (the base plus the buyback) without touching your manabase. The spell costs nothing but tapped bodies, returns to your hand, and leaves another Saproling behind every time it resolves. Then comes the wrinkle that makes it lethal in a single turn: Convoke does not use the tap symbol, so it sidesteps summoning sickness entirely. The fresh Saproling you just made is legal fuel for the next cast on the same turn, which means a large enough board doesn't iterate the loop across turns; it iterates it until you run out of patience or untapped creatures. The army funds its own expansion, and the expansion immediately funds the next cast. The catch sits entirely on the front end: with no board down and the buyback paid out of lands, this is a slow, expensive way to manufacture one token, and the engine only ignites once there are creatures worth tapping. That setup cost is the whole brake on an otherwise free, self-accelerating motion.