If my opponent uses Hextech Ray to kill my only defending unit during a showdown, can I still play action spells (like Hextech Ray) to kill their unit before they can conquer?

Ruling: Yes, you can still play action spells even after your only unit is killed during a showdown. The showdown does not end until both players pass while the state is open, so you can retaliate with spells to kill their unit and prevent conquest.

Sequence:

• Opponent plays Hextech Ray, killing your only unit
• Your unit dies during cleanup (when state changes between open and closed)
• Showdown continues because both players haven't passed
• You can play action spells (like Hextech Ray) to kill their unit
• If their unit also dies, conquest is prevented

Nuances:

• Units with lethal damage die during any cleanup that occurs while they have lethal damage
• Cleanup is triggered when the state changes between open and closed (when there's a chain and when there's not a chain)
• A showdown having zero units on one side does not automatically end the showdown