Whenever equipped creature attacks, it deals damage equal to its power to the player or planeswalker it's attacking.
5/1/2009 This ability triggers and resolves in the declare attackers step. The creature will still deal its combat damage as normal later on during the combat phase.
5/1/2009 When the equipped creature attacks, the first ability will trigger. When that ability resolves, that creature will deal damage equal to its power to player or planeswalker it’s attacking, even if Mage Slayer has left the battlefield or is equipping a different creature by that time.