Thrummingbird

Proliferate was the mechanic that turned a stack of unrelated counters into a single payoff, and this is the cheapest evasive engine for triggering it. The whole design hinges on the connection between flying and the combat-damage trigger: a 1/1 with no evasion would rot in front of any blocker, but a flier that small slips through almost unopposed, and each connection lets you add one more counter to everything you care about. That ratchet effect is the point. One trigger is marginal; a flier that connects every turn compounds. Planeswalkers climb toward ultimates, charge counters fill toward thresholds, +1/+1 counters grow your board, and any poison you have started accumulates toward the ten that ends the game. The frailty of the body is deliberate, because the ability carries the whole pitch: pay two mana for a recurring proliferate that costs nothing once it is online, and accept that the slightest interaction shuts it off. It contributes no counters of its own, so an empty board leaves it inert while a developed one turns each swing lethal. Proliferate has appeared across several later sets, but the cleanest expression of the keyword's strategic axis (small, evasive, repeatable) has rarely been priced this aggressively since.