Invert the Skies is a uncommon Instant card from Eventide (EVE), collector number 155. Current market price: $0.22 (foil $0.70), TCGplayer market prices via Scryfall.
Creatures your opponents control lose flying until end of turn if {G} was spent to cast this spell, and creatures you control gain flying until end of turn if {U} was spent to cast this spell. (Do both if {G}{U} was spent.)