Mage Slayer

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.

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.