Conversion

Rather than destroying the opponent's lands, this rewrites them: every Mountain becomes a Plains, turning off red mana production at the source while leaving the lands themselves untouched. The upkeep tax keeps the rate honest. Pay two white each turn or the lock dissolves, which makes this a commitment rather than a one-shot answer: a resource war fought in white's favorite currency. The design lineage here is the sideboard hoser, a card class Wizards leaned on heavily in the game's first years: narrow, color-pinned, devastating against the one matchup it was built for and inert against everything else. Conversion is white's entry against red alongside cards like Flashfires and Karma, and it represents an approach to format balance the modern game has largely set aside. Cards now tend to do something against a wider field at a worse rate against any single target; Conversion does nothing if your opponent never touches a Mountain, and it can win the game by itself if they lean on one. The land-type-matters design space it occupies (later expanded by Blood Moon, Magus of the Moon, Ruination, and the suite of land-transformation effects that followed) traces back to exactly this kind of effect: an enchantment that wins or loses on a single line of the opponent's deck list.