Mind's Dilation is a mythic Enchantment card from Eldritch Moon (EMN), collector number 70. Current market price: $4.36 (foil $9.08), TCGplayer market prices via Scryfall.
Whenever an opponent casts their first spell each turn, that player exiles the top card of their library. If it's a nonland card, you may cast it without paying its mana cost.