Evolution Vat

The activation tax is doing two jobs at once. The base ability is a Simic counter machine that taps things down while it grows them: pay three, tap a creature, hand it a +1/+1 counter. The trickier half is the temporary grant, which loans the targeted creature a doubling ability that lasts a single turn. A doubling effect that stuck around permanently would snowball every creature on the board into a finisher; gating it to one turn means you have to choose your moment, firing the double on the turn the creature already carries enough counters to make it count. Those counters can come from anywhere: the Vat's own taps over several turns, sure, but also from other counter sources you have stacked onto the same body, since the granted ability doubles whatever is already there. The cost lives on the creature, not on the Vat, so the controller of the targeted permanent decides whether to pull the trigger, an odd wrinkle when the Vat reaches across the table to tap an opposing creature. Later eras tend to fold growth and the explosive payoff into the creatures themselves; this one keeps the engine parked on an artifact and rents out the burst by the turn. It is a deliberate machine that asks for repeated activations before it does anything spectacular, and the expiring clause is exactly what holds the snowball in check.