Who gets to decide who gets saved by Zhonya's Hourglass when multiple units are killed by Elder Drake's ability triggering?

When multiple units die simultaneously—such as when Elder Dragon's play effect deals damage that results in multiple units being killed—the controller of the Zhonya's Hourglass decides which unit is saved.

According to the rules and related RiftJudge FAQs:

• Simultaneous Deaths: When multiple units die at the same time, it is treated as multiple simultaneous events that a replacement effect can apply to.
• Controller's Choice: Rule 373 states that if multiple events occur simultaneously that a replacement effect could apply to, the controller of the replacement effect (Zhonya's Hourglass) decides which event to apply it to first.
• Mandatory Replacement: Zhonya's Hourglass is a mandatory replacement effect. It must be applied to the first event you choose to replace. Once it is applied to one unit, it is consumed (because the gear itself is killed in the process), and it cannot save any other units from that same batch of simultaneous deaths.

Important timing note: As clarified in RiftJudge FAQs (e.g., FAQ #331, #1042), Zhonya's Hourglass must be face-up on the battlefield before the damage is dealt and the units would die. You cannot wait to see which units are dealt damage and then choose to flip the gear to save one; it must already be active to replace the death event.