Acidic Dagger

Oracle Text

4, T: Whenever target creature deals combat damage to a non-Wall creature this turn, destroy that non-Wall creature. When the targeted creature leaves the battlefield this turn, sacrifice Acidic Dagger. Activate this ability only before blockers are declared.

Card Rulings

10/4/2004 Acidic Dagger does not affect the creature if all damage is prevented or redirected.
9/20/2013 If a turn has multiple combat phases, the ability can only be activated before the beginning of the Declare Blockers Step of the first combat phase in that turn.