Whir of Invention [Aether Revolt Prerelease Promos]

Set: Aether Revolt Prerelease Promos Type: Instant Rarity: Rare Cost: {X}{U}{U}{U} Improvise (Your artifacts can help cast this spell. Each artifact you tap after you're done activating mana abilities pays for {1}.)Search your library for an artifact card with mana value X or less, put it onto the battlefield, then shuffle.