Knowledge Exploitation

Tribal Sorcery - Rogue, 5UU (7)
Prowl {3}{U} (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 his or her library.¹
Illus. Darrell Riche

Gatherer Card Rulings [Nov 2009]

• 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 his or her library.

Format Legality	Legal in Vintage, Legacy, Extended, MTGO Classic Banned in Standard
Printings (1/10)	Morningtide (Rare)