When you have a hidden Zhonya's Hourglass at a battlefield with multiple units and one is about to die, are you forced to activate the hidden Zhonya's, or can you choose not to use it?

Ruling: You are not forced to activate a hidden Zhonya's Hourglass. Hidden cards are private information and not in play yet - they may be played for 0 as a reaction, but this is optional.

Nuances:

• Hidden cards have no public information and the game has no way to force you to play them
• If you lose control of the battlefield, hidden cards there are trashed
• For combat damage, you must flip Zhonya's during the showdown BEFORE damage assignment (there is no reaction window during damage assignment)
• For spell or ability damage, you must flip it in response to the spell/ability before the damage happens, not in response to the damage itself