Reiterate

Fork is the obvious ancestor, but Fork copies once and is gone. The real design move is stapling Buyback onto a copy spell, which converts a one-shot trick into a recurring engine: pay the extra three on each cast and the card returns to your hand as it resolves, ready to copy the next thing you point it at. That loop is what put it in every combo conversation it has ever entered. The arithmetic is strict, though, and that strictness is what keeps the engine honest: each iteration costs three to cast plus three to buy back, so an open-ended chain needs a copy target that nets at least six mana per cast (a high-output ritual, a mana-positive instant) before the loop runs free. Anything cheaper just bleeds you out. Left to itself it does nothing; it needs a worthwhile target already on the stack and a payload that pays for the recursion. The license to copy any instant or sorcery, opponents' included, lets the card hold two jobs at once: steal an enemy tutor, double a wrath, or split one big sorcery into two with fresh targets. The window is the constraint, since the copy is made while the original is still on the stack waiting to resolve, never after it has left. Buyback is the hinge that decides, on any given turn, which of those roles Reiterate is going to play.