Mage Slayer
Artifact — Equipment
Whenever equipped creature attacks, it deals damage equal to its power to the player or planeswalker it's attacking.
Equip 
"When the wielder is strong enough, any sword will do." —Kresh the Bloodbraided
Lars Grant-West
Neon Dynasty Commander (NEC)
• #140Rulings
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.
(wotc • 2009-05-01)
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.
(wotc • 2009-05-01)
Rulings
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.
(wotc • 2009-05-01)
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.
(wotc • 2009-05-01)