Intruder Alarm

Builders kept circling this card for years, because it stapled a punishing tax to an engine that runs on a single trigger word. The first clause is a genuine cost: creatures stay tapped through your untap step, so anything you tap to attack or activate stays down until something untaps it. In a fair deck that is crippling. The second clause is where the machine lives, and it turns on a precise distinction: the untap fires whenever a creature enters, not when a spell resolves. That matters because tokens never resolve off the stack; they simply enter. So any repeatable way of putting a creature onto the battlefield becomes a repeatable way of untapping every creature with a tap ability. The cleanest loops close on one other piece. A creature that taps to make a token, Bloodline Keeper minting vampires or Krenko, Mob Boss spitting goblins, goes infinite by itself: it taps, the token enters, the enter-trigger untaps it, and it taps again. No payoff behind the maker; the maker is the payoff. The crippling first clause is exactly the price a combo player is happy to eat while a fair deck cannot afford it. It reads symmetrical, but the untap fires for any creature entering on either side, so a careless opponent's blocker or token can power your loop as readily as your own. That asymmetry hiding inside symmetry is the heart of the card: a one-line tax bolted to a one-line engine, waiting for one more part.