Gadwick, the Wizened

Blue's classic X-for-cards payoffs never left anything on the board: Prosperity refills and vanishes, Blue Sun's Zenith is a spell you cast and forget. Here the reload comes stapled to a legendary Wizard whose second ability turns the entire rest of your deck into a soft Sleep. Every blue spell you cast afterward, cheap cantrips included, taps a nonland permanent an opponent controls, which is the quiet trick of the design: the enters-the-battlefield refill front-loads the cards, and each of those cards can then tap a blocker or a mana source or an untapped threat on its way to the graveyard. That converts a card-advantage engine into a tempo lock, so a controlling blue deck stops being purely reactive and starts pinning the board while it draws. The tap is not conditional on the spell doing anything relevant; it triggers on the cast, so counterspells and card-selection you were already casting arrive with a free tap rider. The cost is in the mana: the reload only pays when X is large, so the card wants to be cast late, and a 3/3 that has already spent its draw trigger is a fragile thing to leave on an empty board. It rewards a deck built to flood the stack with cheap blue spells, at which point the tap does more work over a game than the opening handful of cards ever will.