Mage Slayer

• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)

• WWizards of the Coast2009-05-01
  Official Ruling
  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.
• WWizards of the Coast2009-05-01
  Official Ruling
  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.

Rulings via Scryfall