If I play Hextech Ray on an opponent's minion and they respond with Unyielding Spirit, can I then play a damage spell like Voidseeker or Shakedown to kill the unit before Unyielding Spirit resolves?

Ruling: Action spells cannot be added to an existing chain, so Voidseeker cannot be played in response to Unyielding Spirit. However, Reaction spells can be added to a chain and will resolve before Unyielding Spirit, potentially killing the unit before it gains protection.

Sequence:

• If you play a Reaction spell (like Shakedown) in response to Unyielding Spirit, your spell resolves first
• If the damage kills the unit, it dies before Unyielding Spirit resolves
• If you play an Action spell (like Voidseeker) after the chain resolves, Unyielding Spirit's protection is already active until end of turn, so damage is negated (though you still draw from Voidseeker)

Nuances:

• Actions can start a chain but cannot be added to an existing chain
• Reactions can both start a chain AND be added to an existing chain
• Voidseeker still allows you to draw a card even if the damage is negated