Magewright's Stone

Untap effects are usually printed as one-shot spells; this packages the function as a repeatable, colorless engine attached to a target the deck already wants to use. The restriction is what shapes it: the stone only works on creatures whose activated abilities include the tap symbol, which fences it to tappers, mana dorks, and creatures with effects gated behind tapping. Pay one and tap the stone, and that creature gets to do its thing twice a turn. The classic line is a mana creature: untapping a dork that produces more than the activation cost turns the stone into a net-positive ramp loop, and untapping a creature whose tap ability mills, drains, or pings turns it into a slow inevitability machine. Because the stone itself carries no color and asks only for a generic mana, it slots into any deck running the right kind of creature, which is the quiet appeal of this design: it does not care what colors you play, only what your creatures do when they tap. It is the inheritor of an old idea, the standalone untap effect, rebuilt as a permanent that pays itself off across a game rather than once. The catch is that a summoning-sick creature gives it nothing to work with, and without a legal target it sits idle, so the deckbuilder has to commit to creatures with tap-gated value before the stone earns its slot.