Spelljack is a rare Instant card from Judgment (JUD), collector number 51. Current market price: $1.60 (foil $12.40), TCGplayer market prices via Scryfall.
Counter target spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard. You may play it without paying its mana cost for as long as it remains exiled. (If it has X in its mana cost, X is 0.)