Can you play a hidden Zhonya's Hourglass in reaction to a Deathknell trigger to save the dying unit?

Ruling: No. When a unit with Deathknell is killed, the Deathknell trigger is added to the chain as a pending item before the unit moves to the trash. By the time you have priority to react to the Deathknell trigger, the unit is already dead and in the trash. Playing Zhonya's Hourglass at this point will not save the unit because the death event has already occurred and there is nothing for the replacement effect to replace.

Sequence:

• Unit is instructed to be killed
• Deathknell trigger is added to the chain as a pending item
• Unit is moved to the trash (death completes)
• Deathknell trigger finalizes on the chain
• Priority opens for reactions
• If Zhonya's is played now, it enters play but the unit remains dead

Nuances:

• Zhonya's replacement effect only applies to death events that "would" occur, not events that have already completed.
• During combat, you can still play hidden cards even after your only unit dies because battlefield control is locked until combat ends. Outside combat, losing your only unit at a battlefield causes you to lose control and discard hidden cards before you can react.