Lightning Storm

Most burn spells charge mana for their damage; this one charges cards. The printed figure is fixed and forgettable, because the whole spell is built around a cost paid not in mana but in lands discarded from hand: each land discarded adds two charge counters, and so two more points to X, with any player free to pay while the spell sits on the stack. That last clause is what flips a removal spell into a combo finisher. The kill turn is all-in by construction, which is the conversion rate doing the balancing work: every two points of lethal costs you a card and a land you could have cast spells off of, so the deck that abuses this is one that wants to empty its hand anyway and treats lands as ammunition rather than as resources. There is a real wrinkle in the timing, though: putting the counters on is the effect of the activation, not its cost, so each iteration resolves separately and hands priority back around the table before X actually grows. The "any player may activate" wording cuts both directions, a relic of an older taste for fully symmetric stack interaction: an opponent with lands to spare can discard their own to pump the counters and steal the target, rerouting your lethal spell back at you. It rarely matters, but it belongs to a vein of two-sided stack puzzles the game stopped printing once it decided your spells should stay yours.