Crevasse

Mountainwalk was a threat worth a dedicated answer in the early days, and this is the red mirror's purpose-built fix: an enchantment that does one thing, neutralize the "as though unblockable" clause that let an opponent's hill-dwellers stride past your defenders. It belongs to a Legends cycle where each color got a permanent keyed to disarming the landwalk tied to its own basic land, and that symmetry tells you how early design conceived of a hoser: dedicated, permanent, color-pie-locked, and so narrowly shaped that it does nothing at all in a game where the relevant matchup never shows up. The design is deliberately surgical. It does not destroy the attacker, does not tax the ability, does not touch other forms of evasion; it strips the unblockable clause and leaves everything else in place. The mechanical lineage runs forward from here through every color-hate enchantment and eventually the more flexible "loses all abilities" effects that solved the same problem without committing a whole card to a single keyword. But the ancestor is exactly this: an answer so specifically contoured to one problem that it has no second use. A historical artifact from when mountainwalk was a mechanic you had to actually plan around at the deckbuilding table, rather than a curiosity printed on the occasional creature.