Exile the top X cards of your library. You may cast an instant or sorcery spell with mana value X or less from among them without paying its mana cost. Then put the exiled instant and sorcery cards that weren't cast this way into your hand and the rest on the bottom of your library in a random order.
If you cast a spell with {X} in its cost without paying its mana cost, you must choose 0 as the value of X.
You don’t have to cast an instant or sorcery spell this way. If you don’t, all exiled instant or sorcery cards will be put into your hand.