Rath's Edge

The land that eats itself for damage. The activation is deliberately expensive: four mana, the tap, and a sacrificed land per ping, which prices the ability as a slow grindstone rather than a reach spell. What it really sells is inevitability. A deck with extra lands lying around (or a way to recur them) turns each surplus land into a point of damage that no creature can block and no counterspell answers, since the effect lives on a permanent already in play. That is the design tension worth noticing: it converts a resource everyone treats as flooding into a finisher, but only at a rate slow enough that the conversion never threatens to be efficient. The legendary status is mostly a flavor concession to Rath, the artificial plane the card is named for; mechanically it is a colorless mana source that happens to carry a long fuse. It produces colorless mana with the tap, so it costs you nothing to run as a land while you wait for the late game where the sacrifice clause becomes a closing engine. Cards that fetch lands back from the graveyard, or that make extra lands cheaply, are what move it from a curiosity to a clock. On its own it is patient to a fault; given fuel, it ends games one land at a time.