Artifact — Equipment, 1RG (3)
Uncommon
Whenever equipped creature attacks, it deals damage equal to its power to defending player.
Equip {3}
"When the wielder is strong enough, any sword will do."
—Kresh the Bloodbraided
Illus. Lars Grant-West

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 defending player, even if Mage Slayer has left the battlefield or is equipping a different creature by that time.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern