If Zhonya is hidden at battlefield A and a unit at battlefield B dies in combat, can the hidden Zhonya be revealed to save the unit at battlefield B, or does it only work for units at battlefield A?

Ruling: Zhonya only protects units at the battlefield where it is located, even when hidden. It cannot be used to protect units at a different battlefield.

Sequence:

• If you reveal Zhonya while hidden, it only affects units at the battlefield where it was placed
• As a gear, when unhidden it would go to base at the next opportunity (though this interaction is noted as weird and likely to be corrected)
• If you don't unhide Zhonya and lose control of the battlefield where it's hidden, it goes to trash immediately

Nuances:

• The rules as written create a strange interaction where unhiding Zhonya causes it to eventually go to base, which is acknowledged as likely needing future correction
• If Zhonya remains hidden and you lose the battlefield, it is sent to trash without going to base