Assassinate

The "tapped" clause is the whole conversation here. Black gets unconditional creature destruction at a premium (Murder, Hero's Downfall, Doom Blade and its riders), and the way to push the rate down is to constrain when the removal works rather than what it can hit. Tying the kill to a tapped target does exactly that: it cannot answer a creature held on defense, cannot interrupt a fresh blocker before damage, and asks you to wait until your opponent has committed something to attacking, to an ability, or to a tap-down effect of your own. That makes it reactive in the worst way, since the most threatening creatures are often the ones that never need to tap. In practice the card leans on anything that taps a creature for you (false-block baits, tap effects, even just the natural rhythm of an opponent attacking), so a passive removal spell starts functioning as a punish. It sits in a long line of black removal with a built-in window, and it embodies a design lesson Wizards has returned to repeatedly: that timing restrictions can be as load-bearing as targeting restrictions. The catch is that "tapped" is the cheapest constraint to play around, which keeps the card pinned as a baseline reference point rather than a staple: a clean illustration of how much a single conditional word costs a removal spell.