Mine Collapse

The alternate cost is deliberately turn-locked, and that restriction is the whole balancing act. Five damage to a creature or planeswalker for a single sacrificed Mountain would be a brutal discount if you could ambush with it, so the free line only fires on your own turn. You can still cast it during an opponent's turn to blow out an attacker or answer a combat trick, but then you pay the full . The discount and the flexibility are split apart, and that split is what keeps the rate honest. The free mode is not for turns when your mana sits idle (you would just pay retail then); it earns its keep when you are tapped out or double-spelling, when a spare land becomes removal you could not otherwise afford. Red has a long tradition of free interaction, from Fireblast pitching Mountains to Pyrokinesis exiling red cards, but that tradition is overwhelmingly instant-speed ambush: catch the alpha strike, blow out the combat step, protect the combo on the wrong turn. This one inverts the usual bargain. Red's other free spells buy you the surprise; this one strips the surprise out on purpose, handing you cheap removal for a big blocker or a threatening planeswalker only from your own side of the turn, out of your own flooding manabase, at the moment of least abuse.