Permafrost Trap

The Trap mechanic was the design experiment of building a hate card whose discount only fires when the opponent walks into it, and this one names green creatures as the trigger. Pay full price and it is a four-mana double tap-down with a lockdown rider, an unexciting tempo play priced too high to maindeck. But if a green creature resolved under an opponent's control this turn, the cost collapses to a single blue, and the spell becomes a near-free Frost Breath that also strands two attackers through their next untap. That conditional is the whole proposition: the card is a sideboard answer disguised as a maindeck spell, sized so that against the wrong opponent it is a brick and against the right one it is a blowout. The instant timing matters more than the tap effect suggests. Because the discount keys off a creature entering this turn, the natural window is the opponent's combat, after they have committed a fresh green threat: tap the new arrival and an old one, then watch both sit out a turn while the tempo swings back. It is a punisher in the oldest sense, a card that does nothing until an opponent makes the move it was built to answer, then makes them regret it for one mana. The narrow color hate keeps it honest; a generically cheap version of this effect would warp tempo, so the trigger pins it to a matchup.