Icewrought Sentry

The tapper and the beneficiary live on the same card, which is the quiet engineering trick. Blue has printed Icy Manipulator effects for decades and plenty of creatures that grow when a condition is met, but stapling both halves onto one body means the attack trigger feeds itself: pay the extra mana on the swing, tap the blocker you were worried about, and the untap-to-tap transition pumps the Sentry to a 4/4 for that same combat. Crucially, the second ability keys off you tapping an opponent's untapped creature, not on the creature simply becoming tapped, so it rewards a deck that actively spends tap effects as a resource rather than one hoping outside triggers cash in. What keeps the loop from running away is the sequencing: the pump only fires on the transition from untapped to tapped, so it does nothing against a board already committed to the attack, and the optional payment competes for the mana you would rather hold for interaction. Vigilance is the piece that makes the posture coherent, letting the Sentry attack, tap, and grow while still standing back to block the following turn. It is a midrange tempo tool built around a verb, tapping, that most decks treat as incidental, and its ceiling scales with how many other ways you have to lay hands on an opponent's board.