Mindslaver

No other card hands you an opponent's entire turn. The activation lets you control a target player during their next turn: you see their hand, draw their cards, attack with their creatures into their own board, and make every legal decision the turn offers. The numbers are the leash on all that power: ten total mana committed (six to cast, four plus a tap and a sacrifice to fire) for one stolen turn, with the artifact destroying itself in the process. That is the tension the design resolves. A repeatable version would be unbeatable; a one-time version is a haymaker you have to assemble twice over. The exception is the loop the card is famous for, where a way to return it from the graveyard converts one stolen turn into all of them, and the opponent simply never makes another decision. Constructing that loop is the entire project. Absent it, the effect wins the turn it resolves, then it is gone. The cost is unusually candid about what the effect is worth, which is the reason it has aged into a marquee combo piece rather than a workhorse. It does the single most invasive thing an artifact can do, and the price is set to match.