Etali, Primal Storm

If an exiled card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Any cards not cast, including land cards, remain in exile. They can't be cast on later turns, even if Etali attacks again.

If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, those must be paid to cast the card.