When multiple units die simultaneously in combat and the opponent has Zhonya's Hourglass in play, who decides which unit is saved?

Ruling: The controller of Zhonya's Hourglass decides which unit to save. Multiple units die at the same time regardless of the order damage is assigned, so the Zhonya's controller chooses which one to save.

Sequence:

• During combat, damage is assigned by players in order (attacker first, then defender)
• All damage is dealt simultaneously
• Units don't die when they receive damage, but rather during a specific cleanup step
• When multiple units would die at the same time, Zhonya's controller chooses which to save

Nuances:

• If Zhonya's is hidden at the battlefield, it must be flipped before the showdown ends and damage is assigned, as there is no window after combat to use it at reaction speed
• The order in which damage is assigned does not affect when units die - they all die at the same moment
• "Assigning damage" and "dealing damage" are treated as effectively the same for current game mechanics, though technically distinct concepts