If Zhonyas is hidden at battlefield A and a unit is dying at battlefield B, can Zhonyas be played from hidden at battlefield A to save the unit at battlefield B?

Ruling: Yes, Zhonyas can be played from hidden at battlefield A to save a unit dying at battlefield B.

Nuances:

• Zhonyas's ability is not a play ability, so it isn't restricted to the battlefield where it was hidden
• Play abilities and spells played from hidden have targeting restrictions that limit them to the battlefield they were hidden at
• Zhonyas doesn't target a unit when played; it saves the next unit that dies, so battlefield restrictions don't apply