You have a hidden card on a BF and it's Zhonya's Hourglass and a unit with Deathknell. The unit dies and you don't react with the Zhonya's. Do you take the Zhonya's to base or is it gone?

According to the RiftJudge FAQs regarding hidden Zhonya's Hourglass, if you do not react to save your unit when it dies, the hidden card is discarded.

Here is the breakdown of why this happens:

1. Cleanup and Battlefield Control: When your only unit at a battlefield dies, you lose control of that battlefield.
2. Hidden Card Fate: According to Rule 107.3.d, if a player loses control of a battlefield, any cards in the associated Facedown Zone are removed during the next cleanup.
3. Timing: Because you chose not to use your reaction window to flip and activate the hidden Zhonya's Hourglass, the unit's death finishes resolving, combat (or the game state) progresses, and the cleanup step initiates. Since you no longer have any units remaining at that battlefield, you lose control, and the hidden card is discarded.

As noted in the RiftJudge FAQ #9663, once the death event has occurred and you have passed your opportunity to react, the opportunity to use the Zhonya's is gone. If you fail to play it from its hidden state before control of the battlefield is lost and cleanup occurs, the card is trashed.