Deluge

Most board-wide tappers freeze everything without prejudice; this one carves out flying as an exception that reads like a drawback but functions as a license. The clause is the whole engine: leave fliers untapped and you split the battlefield along its one structural seam, ground stalls while the sky stays mobile. The instant timing is what makes it dangerous, and the defensive use is the cleaner one. Hold it for the opponent's turn before their attack step and you tap their grounded army out of an alpha strike for a price that slips under a counterspell with mana to spare; your fliers, unaffected, are free to block or sit untouched. The offensive case is narrower than it first looks, because the exemption cuts both ways: tapping an opponent's ground creatures on your own turn does nothing to clear a path that fliers already fly over, and the grounded creatures untap on their controller's untap step, so there is no lasting suppression to bank on. Where it does break a race open is when an opponent's blockers can actually answer your air force, the ones with reach: tap those before combat and the fliers swing through a board that should have stopped them. Blue rarely gets to disarm a creature deck on the spot, and the way it does it here is to weaponize the one advantage an evasive flock reliably holds over its earthbound rivals.