Can I play a hidden Zhonya's Hourglass in response to Tasty Faefolk's Deathknell trigger to save the gear, even if Tasty Faefolk was my only unit?

Ruling: Yes. When Tasty Faefolk dies and its Deathknell trigger is placed on the chain, the chain becomes active. Control of a battlefield cannot be lost while there is an item on the chain, so you retain control and can play the hidden Zhonya's Hourglass in response. The gear will enter play at your base, but it will not save Tasty Faefolk, which has already been moved to the trash.

Sequence:

• Tasty Faefolk is killed and moved to the trash
• Deathknell trigger is added to the chain
• The chain is now active (closed state)
• You retain control of the battlefield and receive priority
• You play Zhonya's Hourglass from hidden as a reaction
• Zhonya's Hourglass resolves and enters your base
• The Deathknell trigger resolves

Nuances:

• This works both during and outside of showdowns, as long as the Deathknell trigger creates an active chain
• The gear is saved from being trashed, but the unit cannot be saved because it has already died
• To save Tasty Faefolk with Zhonya's, you would need to play it before the lethal effect resolves