My opponent has a Lonely Poro and a hidden card on Battlefield A. I attack with my Pit Rookie. Both of our units have 2 might. Both players pass and units both die. Then my opponent flips his hidden and shows a Zhonya's and puts it in base. Is this legal? Isn't the hidden discarded because my opponent lost control of the Battlefield?

Question

Accepted Answer

Yes, this is legal.

Normally, if you lose control of a battlefield, any hidden cards there are immediately discarded to the trash. However, because **Lonely Poro** has a `[Deathknell]` ability, its death creates a specific timing window that allows your opponent to save their hidden card.

Here is the exact sequence of how this resolves based on **RiftJudge FAQ #1972**:

1. **Combat Damage:** Both players pass, and combat damage is dealt. **Pit Rookie** and **Lonely Poro** both take lethal damage and die simultaneously.
2. **Deathknell Triggers:** Because Lonely Poro died with no other friendly units at the battlefield, its `[Deathknell]` ability triggers and goes on the chain.
3. **Reaction Window:** The Deathknell trigger opening a chain means the game state is now "Closed." Both players get priority to play `[Reaction]` cards before the trigger resolves.
4. **Flipping Zhonya's:** Your opponent uses this window to play their hidden **Zhonya's Hourglass** as a reaction. 
5. **Zhonya's Resolves:** When played from hidden, Zhonya's Hourglass enters play and immediately recalls to their base (as Gear resides in the base). It does *not* save the Lonely Poro (since the Poro is already dead), but it successfully saves itself.
6. **Control Changes:** After the chain resolves and the game returns to an Open state, the game checks for battlefield control during cleanup. Since your opponent no longer has units there, you gain control of the battlefield.

If the defending unit had been a unit *without* a Deathknell ability, no chain would have opened after combat damage. The game would have gone straight to an Open state, you would have taken control of the battlefield, and the hidden Zhonya's Hourglass would have been discarded to the trash without your opponent getting a chance to flip it.