Primeval Spawn

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.

The spells are cast one after the other during the resolution of Primeval Spawn’s last ability. The one you cast last will be the first one to resolve.

Primeval Spawn’s last ability checks the mana values and types of the spells on the stack, not the mana values and types of the cards in exile. Notably, this means that you may cast the back face of a modal double-faced card or either face of a split card as long as the spells you are casting together have a total mana value of 10 or less.

If an effect causes a card to enter the battlefield as a copy of Primeval Spawn and that permanent wasn’t cast or no mana was spent to cast it, that card will be exiled instead of entering the battlefield.

As long as mana was paid for any cost to cast Primeval Spawn, it enters the battlefield as it resolves. It will not be exiled as it tries to enter the battlefield, even if its cost was reduced or a cost other than its normal mana cost was paid.

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