The Mightstone and Weakstone

Naming an artifact after the two stones Urza and Mishra fought a war over is the kind of storytelling most Magic sets never get to attempt, and the design earns the reference. The stones were always a pair: one that empowered, one that weakened, and the modal enter trigger splits exactly along that line. Choose the Mightstone half and draw two; choose the Weakstone and hand a creature a -5/-5 shrink that outright kills most things and cripples the rest. Either way you keep a two-mana Powerstone rock, colorless mana walled off from nonartifact spells, which is the constraint that keeps a five-mana artifact from doubling as generic ramp: it feeds machines, not spells. The meld line is where the flavor stops being decoration. Pair this with Urza, Lord Protector and the two fuse into a single threat, the mechanical retelling of the brothers' obsession collapsing into one object neither of them could ever share. What sets this apart from most meld cards is that it never needs its other half: draw or removal on entry, ramp thereafter, no dead turns spent hoping to draw the second piece. The meld is upside layered on a card that already pays for itself, which is precisely how you make a set-defining artifact that does not feel like a build-around trap.