Fire Covenant

The pricing here is life, not mana, and that swap is the whole point. The mana cost stays fixed at three; the variable that scales the effect is paid out of your own total, which lets a board sweep cost as little as you can survive. Damage divided among any number of target creatures makes this a multi-target removal spell that splits exactly enough to kill several attackers or blockers in one cast. The design discipline is the life payment: it caps the spell not by mana, but by how much you are willing to bleed, so X scales with the game state rather than your land count. Early, when life is plentiful and creatures are small, X can be enormous relative to the resources committed; late, against a flooded board, you allocate the damage precisely. The instant-speed window changes the calculus a sorcery sweeper cannot offer: hold it through combat, let an opponent commit attackers, then assign damage with full information. Because it targets, hexproof creatures dodge your opponents' side entirely, and indestructible creatures shrug off the damage even when hit; the life cost is a genuine liability against decks racing your total. That tension between flexible board control and self-inflicted damage is the entire card: a removal spell that asks you to spend the one resource that, once gone, ends the game.