Voltaic Key

The untapper, and the quiet enabler behind a long line of artifact engines. The function looks modest: pay one and tap it, and another artifact stands back up. But "untap target artifact" is one of the more dangerous templates in the game, because it converts any artifact with a tap ability into a repeatable engine and turns a mana rock that taps for enough into something that can produce more mana than it cost to deploy. Pair it with a rock that taps for two or more and the math runs positive; chain it with something that untaps it in turn and the loop closes. That open-endedness is exactly why so many of its descendants come fenced in: Manifold Key restricts itself to a similar effect with an upside, and various later untappers carry clauses Voltaic Key never had. This one is unconditional, which is both its appeal and the reason it kept reappearing as the missing piece in combo decks looking for an extra activation or a mana surplus. It does nothing on an empty board and asks you to supply the artifact worth untapping, but in any deck built to exploit a tap ability, that single mana of activation cost is the whole bargain. A combo component dressed as a utility piece, and old enough that the design lesson it teaches (untap effects scale with whatever they target) has shaped how cautiously the mechanic gets printed since.