Playing Master Yi legend, if I have Lonely Poro on a battlefield, and it dies in a showdown, can I play Rengar after drawing the card from the deathknell? Like playing as a reaction?

No, you cannot play Rengar, Trophy Hunter after the Lonely Poro Deathknell trigger resolves and you have drawn your card.

According to the Showdown Timing Flowchart and Chain Rules in the Riftbound rules:

Why this is not possible:

1. Reaction Timing: [Reaction] cards (including units with the [Ambush] keyword, like Rengar, Trophy Hunter) can only be played while there is an active chain or during a window of opportunity created by a Showdown.
2. Chain Resolution: When Lonely Poro dies, its Deathknell trigger is added to the chain. This puts the game into a "Closed" state.
3. Drawing the Card: As the Deathknell trigger resolves, you draw your card. Once that trigger is finished resolving and removed from the chain, the chain becomes empty (assuming no other items were added).
4. No More Window: Once the Deathknell trigger has resolved, the game state returns to "Open." You cannot play a [Reaction] to an effect that has already finished resolving and is no longer on the chain.

The correct timing:

If you want to play Rengar, Trophy Hunter using [Ambush] in this scenario, you must play him in response to the Deathknell trigger while it is still on the chain.

Correct Sequence:

1. Death Event: Lonely Poro dies while alone. The Deathknell trigger is placed on the chain.
2. Reaction Window: While the Deathknell trigger is sitting on the chain (pending resolution), you have a window to play [Reaction] cards.
3. Ambush: You play Rengar, Trophy Hunter using his [Ambush] keyword. Rengar is added to the chain on top of the Deathknell trigger.
4. Resolution (LIFO):
   • Rengar, Trophy Hunter enters the battlefield.
   • The Lonely Poro Deathknell trigger resolves, and you draw your card.

Note: As per RiftJudge FAQ #9841 and FAQ #11170, the "alone" condition is a historical check performed at the moment of death, so even though Rengar enters the battlefield before you draw, you will still successfully draw the card from the Lonely Poro.