Averna, the Chaos Bloom

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.

Cascade triggers when you cast the spell, meaning that it resolves before that spell. If you end up casting the exiled card, it will go on the stack above the spell with cascade.

If a spell with cascade is countered, the cascade ability will still resolve normally.

When the cascade ability resolves, you must exile cards. The only optional part of the ability is whether or not you cast the last card exiled.

If the card 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 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.