Cascade
Keyword actions
Exile from the top until a cheaper spell, then cast it free.
How it works
When you cast a spell with cascade, the cascade ability triggers and goes on the stack above it. You exile cards from the top of your library until you exile a nonland card with a lower mana value than the cascading spell.
You may cast that card without paying its mana cost. Then put the rest of the exiled cards on the bottom of your library in a random order. Because cascade is a triggered ability that resolves first, the free spell resolves before the spell that cascaded.
Common confusions
“Does cascade compare to the cheaper spell's printed cost?”
It compares to the mana value of the spell that has cascade. You're looking for a nonland card with strictly lower mana value.
“Hitting an X spell while cascading.”
A card not on the stack has X = 0, so its mana value is its non-X portion. Cast for free, you must choose X = 0.
“Can the cascaded free spell be countered?”
Yes. Casting for free still puts it on the stack as a normal spell — opponents can respond and counter it.