False Cure

Punishing lifegain is the rare design where the answer waits for the question. Most reactive black spells respond to a creature, a spell, a board state; this one arms a trap and dares the opponent to spring it. Cast it in anticipation of a known lifegain trigger and the gain becomes a guillotine: a single resolved Lightning Helix or a Soul Warden trigger, normally a footnote, suddenly drains the controller for double the swing. The conversion rate is what gives it teeth: two life lost for every one gained means even a modest gain becomes a serious chunk, and a big one is lethal. Its weakness is structural, since it can only punish an opponent who relies on climbing back up; against a player who never gains, the spell simply expires unused. Note the boundary of what it touches: it keys off life gained, not life paid, so cards that ask you to spend life to draw or fix mana fall entirely outside it; only an actual upward swing in life total springs the trap. Read literally, the effect is global and untargeted, so it can backfire on its own caster, which means choosing an instant-speed window where only the opponent stands to gain is half the skill. It belongs to the small tradition of black anti-life tech that treats an opponent's safety valve as a pressure point.