Knowledge Exploitation

Oracle Text

Prowl 3U (You may cast this for its prowl cost if you dealt combat damage to a player this turn with a Rogue.)

Search target opponent's library for an instant or sorcery card. You may cast that card without paying its mana cost. Then that player shuffles their library.

Card Rulings

4/1/2008 You cast the instant or sorcery card as part of the resolution of this spell. It’s cast from your opponent’s library, not your hand. You choose modes, pay additional costs, choose targets, etc. for the spell as normal when casting it. Any X in the mana cost will be 0. Alternative costs can’t be paid.
4/1/2008 If you can’t find an instant or sorcery card that can be legally cast, or choose not to find one, skip that part of the effect. Then the opponent shuffles their library.