Can Zhonya's Hourglass (a hidden gear card in the Poro zone) save Master Yi (on a different battlefield) when Falling Star kills the Poro first and then deals damage to Yi?

Ruling: No, Zhonya's cannot save Yi in this scenario. When the Poro dies, cleanup happens and the hidden Zhonya's is trashed before you can react to save Yi.

Sequence:

• Falling Star resolves, killing the Poro first
• Cleanup happens after the Poro dies, trashing the hidden Zhonya's from the now-empty Poro battlefield
• Falling Star then deals damage to Yi
• Yi dies with no way to save him

Nuances:

• If you react with Zhonya's before the Poro dies, it will save the Poro instead of Yi
• The issue is not that Yi is on a different battlefield (Zhonya's can save units anywhere once played to base), but rather that you lose control of the Poro battlefield before you can time it correctly
• Poro and Yi don't die at the same time with Falling Star; the attacker chooses the order