Can you react with a hidden Zhonya to a unit dying during combat damage assignment, or must it be revealed during the showdown chain before damage is assigned?

Ruling: If Zhonya is hidden when both players pass during showdown, it stays hidden until cleanup finishes. You cannot react to combat damage being assigned - once both players pass, no one can play anything until cleanup finishes.

Sequence:

• Both players pass during showdown
• Combat damage is assigned (no reactions possible)
• Cleanup occurs
• Hidden cards remain hidden throughout this process

Nuances:

• This means with multiple defending units, you don't get to choose which unit to save unless multiple targets are dying
• You can flip Zhonya earlier in the showdown chain before passing, but this reveals it and may affect your opponent's targeting decision