Can a player reveal a hidden Zhonyas to save a unit after that unit has already died?

Ruling: No, the player cannot reveal Zhonyas after the unit dies. The player must activate Zhonyas as a reaction to whatever is killing the unit, before the unit actually dies.

Sequence:

• Opponent casts spell/effect targeting the unit
• Player must reveal and activate hidden Zhonyas in reaction to the spell/effect
• If the unit dies before Zhonyas is activated, it's too late

Nuances:

• Zhonyas cannot be activated on the same turn it was hidden