Kiora, Sovereign of the Deep

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

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

The spell you cast without paying its mana cost is cast during the resolution of the triggered ability. Timing restrictions of that spell based on card type are ignored. It will resolve before the spell that caused the ability to trigger.