Can I chain Zhonyas to my Lonely Poro Deathnell out of Showdown if they use Singularity on Lonely Poro?
Yes, you can react to the Lonely Poro Deathknell trigger with Zhonya's Hourglass, but there is a crucial timing distinction to understand regarding how Zhonya's works.
Can you react?
Yes. When your Lonely Poro dies and its Deathknell trigger is placed on the chain, the game state becomes "Closed." This opens a window where you can play [Reaction] cards. Since Zhonya's Hourglass can be played from hidden (which functions as a [Reaction]), you can certainly play it in response to the Deathknell trigger.
Will it save the Lonely Poro?
No. Zhonya's Hourglass is a replacement effect. For it to save a unit, it must be active at the moment the death event occurs to replace that death with "kill this instead."
By the time the Lonely Poro Deathknell trigger is on the chain, the unit is already dead and has been sent to the trash (or is in the process of being sent there). The Deathknell trigger is simply the result of that death having already happened. Because the death has already occurred, the replacement window has closed, and Zhonya's Hourglass cannot retroactively prevent that death.
How it resolves (LIFO)
- Event: Singularity deals 6 damage to Lonely Poro. Lonely Poro dies, and its Deathknell trigger is added to the chain.
- Reaction: You play your hidden Zhonya's Hourglass. It is added to the top of the chain.
- Resolution:
- Zhonya's Hourglass resolves: It enters the battlefield (or is played as gear), but since there is no longer a death event to replace, it simply enters play.
- Lonely Poro Deathknell resolves: You draw 1 card (assuming it died alone, as per the Deathknell condition).
Summary: You can play the Zhonya's to get it onto the board, but it will not save the Lonely Poro because the death has already finalized by the time the Deathknell trigger is on the chain. You will still get your Deathknell draw, provided the condition of being "alone" was met at the time of death.