Executioner's Swing

The condition is the price. Two mana for a -5/-5 that erases almost any attacker is a rate that would be problematic if it fired on demand, so the design pins it to a creature that has already dealt damage this turn: an attacker that connected, a blocker that traded, a pinger that tapped. That clause turns the spell into a reactive tool rather than a proactive one. You cannot point it at a freshly cast threat sitting back on defense; you wait for it to do something. In practice that makes it a combat instrument first, snapping shut after blocks are declared and damage assigns, when the creature you want gone has already spent its turn committing to the board. The Orzhov color pairing fits the timing exactly: white wants its removal clean and conditional, black wants the kill unconditional, and -5/-5 splits the difference by destroying through indestructible and damage-prevention without technically being destruction. The trade-off is that it asks for patience and a board state where the opponent has already swung or blocked, which is precisely when a defensive deck most wants to be holding interaction anyway.