If Zhonyas is at base, does it have to trigger when a unit dies? And if Zhonyas is hidden, can you leave it hidden till you want to trigger?

Yes, Zhonya's Hourglass is a mandatory replacement effect, and you have flexibility with how you use it when it is hidden.

Here is the breakdown based on the official rules and FAQs:

1. If Zhonya's is at your base

Yes, it must trigger. Zhonya's Hourglass is not a triggered ability; it is a mandatory replacement effect. If a friendly unit would die, the effect must be applied. You cannot choose to let the unit die to save the Zhonya's for a later turn.

• Nuance: If multiple friendly units would die simultaneously (e.g., from a single effect), you choose which one to save, but you cannot choose to save none of them (Rule 373, RiftJudge FAQ #5836).

2. If Zhonya's is hidden

Yes, you can leave it hidden until you choose to trigger it.

• It is optional to play: While the effect is mandatory once it is in play, a hidden card is private information and is not yet "in play." You are not forced to reveal and play a hidden Zhonya's when a unit dies. You can choose to let the unit die and keep the Zhonya's hidden for a future, more critical moment (RiftJudge FAQ #3763).
• Timing: You can reveal and play a hidden Zhonya's at any time you have priority to play a reaction (RiftJudge FAQ #9010).
• The "Gotcha": If you choose not to use it and your unit dies, you must be careful about battlefield control. If that unit was your only unit at that battlefield and the battlefield becomes uncontested, you will lose control of it during the cleanup step. If you lose control of the battlefield, any hidden cards there are trashed (RiftJudge FAQ #7086, #2411).

Summary of Key Mechanics:

• Mandatory vs. Optional: The effect of Zhonya's is mandatory once it is active, but the act of playing a hidden card is always optional.
• Hidden Cards: They are not in play and are private information. You are never forced to reveal them.
• Battlefield Control: If you are relying on a hidden Zhonya's to save a unit, ensure you don't lose control of the battlefield before you have the chance to react, or the card will be lost.