Quandrix, the Proof

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.

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

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

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

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.