The first spell you cast each turn has cascade. (When you cast your first 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 of your library in a random order.)
5/1/2009 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.
5/1/2009 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.
5/1/2009 If a spell with cascade is countered, the cascade ability will still resolve normally.
5/1/2009 You exile the cards face-up. All players will be able to see them.
5/1/2009 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.
5/1/2009 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.
5/1/2009 Maelstrom Nexus affects the first spell you cast each turn, not just the first spell you cast on your own turn.
5/1/2009 If the first spell you cast in a turn already has cascade, both cascade abilities will trigger separately. Deal with them one at a time: First one cascade ability will resolve, and you’ll cast the applicable card if you want. That new spell will resolve. Then the other cascade ability will resolve in the same way. Finally, the original spell will resolve.
5/1/2009 This effect takes into account spells that were cast earlier in the turn that Maelstrom Nexus entered the battlefield, including any spells still on the stack. If you’ve already cast any spells that turn (including Maelstrom Nexus itself), this ability won’t give any of your spells cascade that turn.
5/1/2009 A spell’s converted mana cost is determined solely by the mana symbols printed in its upper right corner. If its mana cost includes , take the chosen value of X into account. Ignore any alternative costs, additional costs, cost increases, or cost reductions.
4/18/2017 The converted mana cost 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.