Manifold Key

The untap ability is the entire reason this exists, and it descends from a small family of artifacts built to double up on another artifact's tap ability. Point it at a mana rock and that rock produces a second time this turn; point it at a tap-to-draw or tap-to-mill engine and you have squeezed two activations out of a thing meant to give one. The target restriction matters: it only reaches artifacts, so a tapped land or a vigilance-less creature that just attacked is off-limits, and the deck has to supply the artifact worth doubling. Because the untap costs plus the tap of the Key itself, it is a once-per-turn boost, not an open-ended crank: one extra untap per turn cycle, and the payoff is only as large as whatever artifact you aimed it at. That ceiling on frequency is what stops the effect from spiraling. It generates nothing on its own, so it belongs in a shell that has already assembled something worth doubling, and it rewards you exactly in proportion to how good that target is. The second mode, and the same tap to make a creature unblockable, is a floor for shells that never assemble an untap target: a fair rate to push a threat through rather than leave the artifact inert. The shared tap forces a lane each turn, and a deckbuilding question about which one you are actually here for.