If a Scuttle at a bf dies and Zhonyas is hidden, can you reveal Zhonyas after the Scuttle dies or does the reveal have to happen before it dies?

To answer your question, it depends on whether the battlefield is contested and whether there are triggered abilities (like Deathknell) occurring as the unit dies.

According to the RiftJudge FAQ #7086 and #8150, you generally cannot react to combat damage being assigned. Here is the breakdown of how this interaction works:

1. If your unit dies to combat damage (No other effects)

• No, you cannot reveal Zhonya’s after.
• If your unit dies to combat damage, both players pass during the showdown, combat damage is assigned, and you proceed immediately to cleanup.
• Because hidden cards gain Reaction speed, you could have flipped (revealed) the hidden Zhonya's Hourglass at any time before both players passed during the showdown. Once both players have passed, no further reactions are allowed until after the cleanup phase.

2. If your unit dies and there is a Deathknell trigger

• Yes, you can react.
• If your unit (like a Scuttle Crab) has a Deathknell ability, that ability triggers when the unit dies. Triggered abilities use the chain.
• When a Deathknell trigger is placed on the chain, this creates a "Closed" state where players have priority. You can use this priority window to reveal your hidden Zhonya's Hourglass as a reaction to the Deathknell trigger (RiftJudge FAQ #7086, #1972).

Summary of Key Mechanics:

• Hidden Cards: A hidden Zhonya's Hourglass has Reaction speed, meaning it can be revealed any time you have priority to play a reaction (RiftJudge FAQ #9010).
• Replacement Effect: Once you reveal and play the Zhonya's Hourglass, its ability is a mandatory replacement effect. It must save a unit if one is dying (RiftJudge FAQ #9690).
• Battlefield Control: Be careful: if your unit dies and you lose control of the battlefield before you have the chance to react, your hidden card will be discarded during the cleanup step (RiftJudge FAQ #7086).

In short: You cannot wait for the unit to be dead and gone to decide to save it; you must reveal the Zhonya's while you still have an active priority window (e.g., during the showdown chain or in response to a Deathknell trigger). Once combat damage is final and no triggers remain, you have missed your window.