Averna, the Chaos Bloom — ruling 1

The mana value of a split card is determined by the combined mana cost of its two halves. If cascade allows you to cast a split card, you may cast either half but not both halves.

Averna, the Chaos Bloom — ruling 2

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.

Averna, the Chaos Bloom — ruling 3

Due to a 2021 rules change to cascade, not only do you stop exiling cards if you exile a nonland card with lesser mana value than the spell with cascade, but the resulting spell you cast must also have lesser mana value. Previously, in cases where a card's mana value differed from the resulting spell, such as with some modal double-faced cards or cards with an Adventure, you could cast a spell with a higher mana value than the exiled card.

Averna, the Chaos Bloom — ruling 4

A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.

Averna, the Chaos Bloom — ruling 5

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.

Averna, the Chaos Bloom — ruling 6

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.

Averna, the Chaos Bloom — ruling 7

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

Averna, the Chaos Bloom — ruling 8

You exile the cards face up. All players will be able to see them.

Averna, the Chaos Bloom : règles, fonctionnement et interactions (MTG)

Averna, the Chaos Bloom is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As you cascade, you may put a land card from among the exiled cards onto the battlefield tapped. (Do this after the last card is exiled but before deciding whether to cast a nonland card.)

How it works : Averna, the Chaos Bloom

Original English text — Wizards of the Coast / Scryfall

#1June 18, 2021· Wizards of the Coast
The mana value of a split card is determined by the combined mana cost of its two halves. If cascade allows you to cast a split card, you may cast either half but not both halves.
No other card is named in this ruling.
#2June 18, 2021· Wizards of the Coast
If the card has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
No other card is named in this ruling.
#3June 18, 2021· Wizards of the Coast
Due to a 2021 rules change to cascade, not only do you stop exiling cards if you exile a nonland card with lesser mana value than the spell with cascade, but the resulting spell you cast must also have lesser mana value. Previously, in cases where a card's mana value differed from the resulting spell, such as with some modal double-faced cards or cards with an Adventure, you could cast a spell with a higher mana value than the exiled card.
No other card is named in this ruling.
#4June 18, 2021· Wizards of the Coast
A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.
No other card is named in this ruling.
#5June 18, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#6June 18, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#7June 18, 2021· Wizards of the Coast
If a spell with cascade is countered, the cascade ability will still resolve normally.
No other card is named in this ruling.
#8June 18, 2021· Wizards of the Coast
You exile the cards face up. All players will be able to see them.
No other card is named in this ruling.
#9June 18, 2021· Wizards of the Coast
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.
No other card is named in this ruling.
#10November 10, 2020· Wizards of the Coast
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.
No other card is named in this ruling.
#11November 10, 2020· Wizards of the Coast
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.
No other card is named in this ruling.