What it does
When you cast a spell with cascade, you exile cards from the top of your library until you hit a nonland card with a lower mana value. You may cast that card for free (without paying its cost), then put the rest on the bottom of your library in random order.
The biggest misconception: you choose whether to cast the revealed card, but you can't pay to "upgrade" it—it's cast as-is or not at all. Also, cascade looks at mana value, so X counts as 0 in the library, and only the cascade spell's own mana value sets the threshold (not its total cost paid). Importantly, the cascade trigger goes on the stack above the spell that caused it, so the free spell resolves first.
In Commander, cascade is a value engine—free spells generate huge tempo and card advantage. Watch for cascading into another cascade spell to chain effects, and note that counterspells targeting your original cast won't stop the free spell that already resolved.













