If a unit with Deathknell is moved by Charm from a battlefield with a hidden Zhonya's Hourglass to another battlefield where it dies in combat, can you flip the Zhonya's in response to the Deathknell trigger to recall it to base without saving the unit?

Ruling: No. You cannot both let the unit die (to trigger Deathknell) and save the hidden Zhonya's Hourglass to your base in this scenario. If you don't react to Charm, the battlefield becomes uncontrolled when your unit leaves, and the hidden Zhonya's is discarded during cleanup. If you do flip Zhonya's in response to Charm, it goes to your base but will automatically trigger its replacement effect to save your unit when it would die in combat, preventing the Deathknell trigger.

Sequence:

• If you don't flip Zhonya's: Charm resolves, your unit moves, the battlefield becomes uncontrolled, cleanup discards the hidden Zhonya's
• If you flip Zhonya's as a reaction to Charm: Zhonya's goes to base, Charm resolves, combat occurs, Zhonya's replacement effect saves the unit (no Deathknell)

Nuances:

• If combat occurred at the same battlefield where Zhonya's is hidden, you would maintain control throughout combat and could flip Zhonya's after the unit dies (since the death event already occurred, Zhonya's wouldn't save it)
• Zhonya's Hourglass uses a mandatory replacement effect that applies automatically when active at your base