Karn's Sylex is a mythic Legendary Artifact card from Dominaria United Promos (PDMU), collector number 234p. Current market price: $1.82 (foil $0.96), TCGplayer market prices via Scryfall.
Karn's Sylex enters tapped. Players can't pay life to cast spells or to activate abilities that aren't mana abilities. {X}, {T}, Exile Karn's Sylex: Destroy each nonland permanent with mana value X or less. Activate only as a sorcery.