Thelon's Curse is a rare Enchantment card from Fallen Empires (FEM), collector number 77. Current market price: $0.63, TCGplayer market prices via Scryfall.
Blue creatures don't untap during their controllers' untap steps. At the beginning of each player's upkeep, that player may choose any number of tapped blue creatures they control and pay {U} for each creature chosen this way. If the player does, untap those creatures.