Nightmare's Thirst

The lifegain here is not incidental upside; it is the throttle on the removal. Most one-mana black removal that scales does so off an external counter (creatures in your graveyard, swamps you control, devotion), but this one feeds itself a single point of life and lets that point do the work. On a bare board, that nets a -1/-1: enough to clip a mana dork or a one-toughness aggro creature, and nothing more. The card's ceiling lives entirely outside its own text, in whatever else you have gained life with this turn before it resolves. Stack it after a lifelink swing, a fetch-and-gain effect, or a soul-sister trigger and the same one black mana suddenly answers something genuinely large, because X reads the whole turn's lifegain, not just its own point. That dependency is the constraint that pays for the rate: a flat one-mana instant that kills a 4/4 would be undercosted, so the design gates the big numbers behind a separate investment you have to assemble first. It is removal built for a deck already counting its lifegain triggers, where each incidental point of life doubles as a downpayment on an instant-speed answer. Left on its own, it is a marginal shrink spell that trades a card for a point of life and a -1/-1; wired into a lifegain shell, it becomes a scaling kill spell that costs almost nothing to hold up.