Aether Vial

Mana never touched it after the first one. That is the whole trick: every creature it deploys arrives without paying its cost, at instant speed, and the only ongoing demand is a tap symbol and the patience to let the counters climb. The counters tick up by choice, one per upkeep if you want them, so the artifact rewards a deck built on a single mana-value tier (the one-and-two-drop bands of linear tribal aggro, where a Vial parked on two puts small bodies onto the battlefield without ever casting them). The constraint that pays for free creatures is the counter math: you can only put down what matches the dial, and because the dial only advances when you choose to add a counter, you control the tier but never get more than one increment a turn. A deck flat in costs stays in tune; a deck top-heavy in costs strands the Vial on the wrong number. That tension is why the card has always lived in tribal shells rather than midrange piles. The activated ability still uses the stack and can be answered before it resolves, but the creature it produces is never a spell, so countermagic aimed at casts does nothing once the ability resolves: enters-the-battlefield triggers fire at instant speed in response to an opponent's play, and a board wipe becomes a one-turn rebuild. The body of the deck does the work; the Vial just makes the deployment free and the timing unfair.