Jeleva, Nephalia's Scourge

If the card has {X} in its mana cost, you must choose 0 as its value.

If Jeleva leaves the battlefield, the exiled cards will remain exiled indefinitely. If Jeleva enters the battlefield again, it won't be associated with the cards the "other" Jeleva exiled. The new Jeleva will exile a new set of cards. Only those cards can be cast using Jeleva's last ability.

The card is cast before blocking creatures are declared.

If you cast an instant or sorcery card using Jeleva's last ability, you do so while that ability is resolving. If you choose not to (or you can't), you won't get a chance to cast it later unless the ability triggers again.

If you cast a card "without paying its mana cost," you can't pay alternative costs such as overload costs. However, you can pay additional costs such as kicker costs. If the card has mandatory additional costs, you must pay those.