Spellfall

As you cast this spell, you may pay {x}.
If you do, its mana value becomes X+2.

As long as Spellfall is on the stack,
spells you cast have cascade.

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.)