If you have a hidden Zhonya's with one unit, can you let the unit die and reveal Zhonya's afterward to return it to base without it playing for the unit's death?

Ruling: You can only reveal a hidden Zhonya's after your unit dies if the battlefield is contested AND there is a deathknell ability (or similar chain-forming effect) that gives you an opportunity to react. If your only unit dies to combat damage with no deathknell triggers, you have no opportunity to react before combat resolves, and the hidden card will be discarded when you lose control of the battlefield.

Sequence:

• Unit dies to combat damage
• If battlefield is contested AND a deathknell trigger exists, you can react during the chain and reveal Zhonya's
• If battlefield is not contested OR no deathknell exists, cleanup occurs immediately
• Hidden cards are discarded when battlefield controller loses control during cleanup

Nuances:

• Facedown cards aren't gear, spells, or units - they're just facedown cards
• Gear (including Zhonya's when revealed) is always played to base, never to battlefield
• During contested battlefields, control can't change, which prevents the cleanup step from discarding hidden cards
• This applies during both combat and showdowns