Thieves' Fortune

Prowl is a static ability that functions on the stack. “Prowl [cost]” means “You may pay [cost] rather than pay this spell’s mana cost if a player was dealt combat damage this turn by a source that, at the time it dealt that damage, was under your control and had any of this spell’s creature types.” Casting a spell for its prowl cost follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.