If you control a battlefield with Zhonya hidden and lose control of the battlefield in a showdown, can you choose not to activate the hidden ability and return Zhonya to base, or must you discard it?

Ruling: If you have a deathknell ability, you can respond to that ability with Zhonya's hidden ability to send Zhonya back to base. Without a deathknell ability, Zhonya will be discarded when you lose the battlefield.

Sequence:

• Combat damage occurs
• If you have a deathknell ability, it triggers
• You get priority to respond to the deathknell trigger with Zhonya's hidden ability
• Zhonya returns to base
• Opponent gains control of the battlefield

Nuances:

• Deathknell is the only way to get priority in the window after combat damage but before the opponent gains control of the battlefield
• Without deathknell, you go straight to the opponent controlling the battlefield without getting a chance to respond
• You cannot use other reactions to activate Zhonya's hidden ability in this situation