Death Wind

The X/X shrink effect has always been black's answer to a problem most of its removal struggles with: scaling. A fixed-damage spell gets stranded against a fattened body, but pumping X here lets the same card kill a token early and a finisher ten turns later, paying only the difference. The instant timing is what gives it teeth. Held up at end of step or waiting on a combat trick to resolve, it lets you size the shrink to whatever the board actually presents rather than committing to a number on your own turn, and because it reduces toughness rather than dealing damage, it answers indestructible and high-toughness threats that burn cannot. The flexibility is paid for in tempo and information: every point of X is a point of mana telegraphed, and at the high end the card costs as much as the creature it is killing, so it tends to trade up only when the opponent has overcommitted to a single fat threat. This is the workhorse template black has reprinted in many shapes (Disfigure trades the scaling for a flat, cheap line; others bolt on lifegain or card draw), and Death Wind sits at the unembellished center of that family: no rider, no graveyard clause, just toughness reduction priced exactly to what you want dead.