Glaciers

Convert every Mountain into a Plains and you have removed nothing from the table; you have rewritten what an opponent's lands tap for. A red deck leaning on basic Mountains watches its red sources turn white, its burn spells choke for color, and its mountainwalk creatures lose the land type they were built to exploit. It is a single-color manabase assassin wearing the body of a four-mana enchantment. The upkeep clause is what keeps it from being a free permanent lock: every turn you re-pay to keep the conversion live, and the turn you cannot, the enchantment hits the graveyard and the Mountains revert. That pay-or-sacrifice frame is pure Ice Age design philosophy, kin to the cumulative-upkeep cards that priced their effects as a recurring tax you had to keep feeding. Its glaring weakness is also its premise: against any deck not built on Mountains it sits inert, a blank that does nothing while it strangles the one opponent whose mana it can reach. The card belongs to a vanished design instinct, one that treated an opponent's basic lands as fair territory to hijack at four mana: a white-blue counterpunch aimed squarely at the red aggression of its day.