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 Primeval Spawn is exiled by its first ability as it tries to enter the battlefield, it goes directly to the exile zone without entering the battlefield first. Since it never enters or leaves the battlefield, its last ability won’t trigger.

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.

Conversely, if an effect would cause Primeval Spawn to enter the battlefield as a copy of another permanent, it will enter the battlefield as a copy of that permanent whether or not it was cast or mana was spent to cast it.

If a creature token that is a copy of Primeval Spawn would be put onto the battlefield, it’s put into exile instead and then ceases to exist. Creature tokens are never cast, even if the spell that created them was.

If an effect from a static ability would remove all abilities from Primeval Spawn if it were on the battlefield (such as that of Humility), then its first ability won’t cause it to be exiled. It will enter the battlefield as normal.

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.

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.

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.

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.