Etali, Primal Storm is a rare Legendary Creature — Elder Dinosaur card from Rivals of Ixalan Promos (PRIX), collector number 100p. Current market price: $3.98 (foil $2.35), TCGplayer market prices via Scryfall.
Whenever Etali attacks, exile the top card of each player's library, then you may cast any number of spells from among those cards without paying their mana costs.