Cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.)
1/5/2009: 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.
1/5/2009: When the Cascade ability resolves, you must exile cards. The only optional part of the ability is whether or not your cast the last card exiled.
1/5/2009: If a spell with Cascade is countered, the Cascade ability will still resolve normally.
1/5/2009: You exile the cards face-up. All players will be able to see them.
1/5/2009: If you exile a split card with Cascade, check if at least one half of that split card has a converted mana cost that’s less than the converted mana cost of the spell with cascade. If so, you can cast either half of that split card.
1/5/2009: If you cast the last exiled card, you’re casting it as a spell. It can be countered. If that card has cascade, the new spell’s cascade ability will trigger, and you’ll repeat the process for the new spell.
1/5/2009: After you’re done exiling cards and casting the last one if you chose to do so, the remaining exiled cards are randomly placed on the bottom of your library. Neither you nor any other player is allowed to know the order of those cards.