Callidus Assassin

Clone effects usually run into a problem of purpose: the copy is one more body on a crowded board, indistinguishable from what it imitated and doing nothing except existing. Polymorphine turns the copy into an execution. Entering as a duplicate of any creature on the battlefield, it carries a stapled destroy clause that fires on its own name, which means it does not just impersonate a creature: it comes in and kills the original outright. The naming restriction is the whole mechanism. Because the trigger only hits a creature sharing this creature's name, and the name is whatever you just copied, the assassin can always find its mark, and it can never turn the blade on you unless you copy something you also control by that name. Flash is what makes the trick lethal rather than symmetric: held up during an opponent's turn to strike once their creature is on the battlefield, it answers a threat with its own reflection at instant speed, a removal spell disguised as a mirror. Arriving tapped is the tax that keeps it honest, since the copy cannot block or attack the turn it lands, so you pay for the assassination in tempo rather than pocketing a free body and a kill in the same breath. It is a Clone rebuilt as targeted removal, where the copy is not the point but the delivery system for the kill it copies its way into.