Cosmium Blast

Four damage for two mana at instant speed would be flagrantly undercosted as red burn, so white buys the rate by shackling it to combat: the target has to be attacking or blocking, never a creature sitting home on defense. That single restriction converts a would-be proactive kill spell into a reactive one, live only after your opponent commits to an attack or you announce blocks. It is a genre white has leaned on for years: combat-locked burn white is allowed to play precisely because the creature has stepped out of a defensive posture and made itself a legal target. Four damage is generous for the color, clearing most midrange bodies and even some larger threats, and there is no sorcery-speed clause to soften the deal; the spell holds through the declare-attackers step and fires the instant a creature is announced, or waits until blocks are declared to punish an alpha strike. The price for the discount is that you surrender timing control to your opponent's combat decisions. A fresh threat the turn it lands is untouchable until it attacks or blocks, and a creature that never commits stays safe forever. This is removal for decks that expect to be attacked, or that want to open a lethal swing by ambushing a blocker mid-combat, rather than a clean answer you can point wherever you like.