Not on My Watch

The condition is the whole trade: exile is a permanent, regenerate-proof, no-second-life answer that white rarely hands out at instant speed for two mana, and the price is that the creature has to be attacking. That restriction does more design work than it looks like. It cannot be a proactive removal spell, cannot pick off a mana dork or a combo piece on your own turn, and cannot answer a threat that simply sits back on defense. It is reactive by construction, offering a rate the color otherwise guards jealously (compare the tighter conditions white usually attaches to permanent exile) in exchange for surrendering all initiative. The waiting buys the cleanest possible result: no lifegain granted, no card returned, no body left to recur. The window is generous within combat but only within combat: a creature counts as attacking from the moment it is declared through blocks, damage, and end of combat, so this can wait out a blocker assignment or answer a survivor after damage, but it is dead the instant the swing is called off. That gating puts it among white's "punish the aggressor" effects, the ones that ask you to let the attack happen before you profit, trading tempo flexibility and proactive reach for an answer that resolves the problem outright.