Karlov's Crossbow (CA06a) [Unknown Event]

Whenever equipped creature attacks, sacrifice Karlov's Crossbow and destroy target creature an opponent controls. Equip {1} Forecast — {0}, Reveal Karlov's Crossbow from your hand. For the remainder of the game, spells you cast named Karlov's Crossbow cost {1} less to cast. (Activate only during your upkeep and only once each turn.)