Beamsplitter Mage

The conditional buried in the trigger is the whole engine: a spell only doubles if it targets this creature and you control another creature the spell could legally copy onto. That second clause turns a humble combat trick into a build-around. Point a pump spell at the Mage and you get two pumped creatures; point removal at it (unusual, but legal) and the copy redirects to a second target. The redirection is automatic and forced, so the deckbuilding job is to fill your spellbook with single-target instants and sorceries worth casting twice and to keep a second body on board for the copy to land on. It inverts the usual copy tax: instead of paying extra to split a spell, you cast a one-target spell at the "wrong" creature on purpose and let the trigger pay you. The intervening 'if' clause is where the fragility lives. It checks whether you control another valid target both when the ability triggers and again when the trigger itself resolves, which happens before the original spell has resolved; if your second creature dies in that window, the copy never gets made. And nothing here dodges priority: the copy goes on top of the stack, players get a window to respond to it, and then another window to respond to the original underneath. The engine rewards cheap, repeatable single-target effects over one expensive payoff, and it goes cold the moment your board thins out.