With the new ruling on split cards, if I cascaded into Destined // Lead (say with Maelstrom Wanderer), would i be able to play either side? What if I cascaded into Turn // Burn which has a total CMC of 5 and has fuse. Do I have the option to cast Turn, Burn, or both?
“Cascade” means “When you cast this spell, exile cards from the top of your library until you exile a nonland card whose converted mana cost is less than this spell’s converted mana cost. You may cast that card without paying its mana cost. Then put all cards exiled this way that weren’t cast on the bottom of your library in a random order.”
(This is the only part of the interaction that is affected by the rules change.) So with Cascade, this means that the combined converted mana costs of both halves of the card need to be less than the CMC of the spell that’s the source of the Cascade trigger. In your example, Destined // Lead (6) and Turn // Burn (5) both have a lower CMC than Maelstrom Wanderer (8) and so they satisfy this requirement.
A spell with Aftermath, such as Lead, can only ever be cast from your graveyard. When you resolve a cascade ability, the spell you cascade into is being cast from exile. Exile is not the graveyard and so you cannot cast the half of the card with Aftermath. In your example, you’ll only be able to cast Destined and not Lead.
You can only cast both halves of a split card with Fuse if you’re casting it from your hand. When you Cascade into such a card, you’re casting it from exile, which isn’t your hand, and thus you can only cast one half or the other but not both. In your example, you’ll be able to cast Turn or Burn, but not both.
