Honor-Worn Shaku

The untap clause is where the whole rock lives. As a plain mana source it produces one colorless and stops, which would be unremarkable, but the second ability turns every untapped legendary permanent you control into a refill button: tap a legend, untap the artifact, tap it again. The genius is how loosely "legendary permanent" reads. It does not have to be a creature, it does not have to do anything when tapped, and tapping it costs nothing but leaving that permanent tapped for the turn. In a board built out of legendary lands, legendary artifacts, planeswalkers, and a stack of legendary creatures, a single rock starts generating colorless mana proportional to how committed your deck is to the legendary supertype. This is ramp wearing the disguise of a humble mana rock: it does nothing in a normal deck and scales sharply in one that treats "legendary" as a deckbuilding constraint rather than an occasional keyword. The colorless-only output is the ceiling on its abuse, pointing the card toward eldrazi, artifact-heavy ramp, and activated-ability sinks rather than splashy multicolor spellcasting. It comes from a design era fixated on legends, one that asked you to flood the board with them and rewarded you for it, and it has aged into one of the cleaner engines of that idea: no upkeep cost, no downside, just a question of how many legendary permanents you can keep untapped at once.