You Cannot Pass!

The gate is combat, and specifically combat that ran through a legendary creature. For one white mana, the destruction itself asks nothing extra of the target: no color clause, no toughness cap, just gone. What it asks instead is that the creature blocked or was blocked by a legend this turn, which routes a Terror-grade kill spell through the battlefield rather than the stack and hands white a premium rate in a color that has historically paid full price to remove a creature. The clever seam is the "this turn" wording. It is a condition on the target, not a trigger tied to the legend, and it does not require your legend to still be alive when you cast the spell. Attack with your legend, let two blockers gang up on it and trade, then point this at whichever survivor you like once combat resolves, even if your legend died in the exchange. The condition, though, is two-sided and cannot be manufactured: an opponent who refuses to block your legend, or refuses to attack into it, never opens the window. The spell rewards a board built around a marquee legend and punishes engagement with it in combat, but it will sit dead in hand against an opponent who simply declines the fight. Cheap and decisive when the setup is there, inert when it is not.