Mage Hunters' Onslaught
Unconditional four-mana removal is a fair rate, and the design charges a small premium for a rider that most decks casting it never collect on. Destroying any creature or planeswalker for two black and two generic is the clean answer that runs through black's midrange history; the second clause is what changes the card's job. Because a block-tax lands on every creature that gets in the way once the spell resolves, it rewards a controller who is already the beatdown: cast it on your own turn, blow open the board, clear a would-be blocker, and let the trigger punish whatever the opponent throws in front of the rest of your attackers. As a sorcery, you'll want to cast it in your precombat main phase, which is exactly where an aggressive black deck wants to spend four mana anyway. The catch is that the kill spell and the tax reward opposite game states from one card: on an empty board it is a bare removal spell, and against a defender with nothing to swing at it collects no life at all. The block-tax only pays out when you have a threat worth attacking with and the opponent is forced to trade down. That is the doubling of purpose the price is buying: an answer to one problem now, and a penalty on defense that folds your removal into the offense the same turn you cast it.
