Averna, the Chaos Bloom

If you don't cast the nonland card you reveal, or if you don't reveal any nonland cards with lesser mana value, you can still put a land card onto the battlefield before you finish cascading.

Averna's ability isn't the same as playing a land. You may do this even if you've already played a land during your turn and even if it isn't your turn at all.

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