When Unchecked Power deals damage to all units and the defending player has Zhonya's Hourglass, who chooses which unit is saved - does the attacker choose the first target hit, or does the defender choose which unit to save?

Ruling: The controller of Zhonya's Hourglass (the defender) chooses which unit to save.

Sequence:

• Unchecked Power deals damage to all units at all battlefields simultaneously
• All units would die at the same time
• Zhonya's Hourglass is a replacement effect that replaces a damage event that would kill a unit
• Since damage is dealt simultaneously, the Zhonya's controller chooses which kill event to replace

Nuances:

• Unchecked Power doesn't target units or battlefields since it affects all units at all battlefields
• Whether the spell targets or not doesn't matter for this interaction
• Zhonya's Hourglass replaces the event of damage that would kill a unit (not the event of being killed as the card text suggests)