If Zhonya's is hidden on a battlefield, does it automatically trigger when any unit dies, or can you choose which unit to save after seeing which one is targeted?

Ruling: A hidden card is never forced to be used and you can use it whenever you would play a reaction. You can choose which unit to save with Zhonya's as long as you activate it at the appropriate reaction timing.

Nuances:

• If all units would die simultaneously from combat damage, you must activate Zhonya's before combat damage is assigned
• If units die in separate combats or at different times, you can wait to see which unit is threatened before deciding to use Zhonya's