Does Deathknell trigger at the same time?

The answer depends on whether the units with Deathknell die due to the same event (like a board wipe or combat damage) or different events.

1. If they die from the same event: Yes, they trigger simultaneously.

When multiple units die simultaneously (such as during the Combat Cleanup phase or from a spell that kills multiple units), their Deathknell abilities are all generated as Pending Items on the chain at the same time.

According to Rule 808.2.a and Rule 383.3.d:

• The controller of the abilities chooses the order in which to place those triggers onto the chain.
• If multiple players each control units that died, they order their triggers starting with the Turn Player and proceeding in turn order.
• Once ordered, they resolve in Last-In, First-Out (LIFO) order.

2. If they die from different events: No, they trigger separately.

If one unit dies during the resolution of a spell (or combat), and another unit dies later (due to a different effect or another part of the same spell), they trigger at different times.

• Each death event creates its own individual Deathknell trigger.
• If a unit dies during the resolution of a spell, its Deathknell trigger is finalized and added to the chain immediately, and it will resolve before the remaining parts of the spell or any other previously pending items continue (see RiftJudge FAQ #3119).

Important Timing Distinctions

• Triggering vs. Resolving: The event of the unit dying is what triggers the ability. However, the resolution of that ability depends on the game state.
• Combat Cleanup: In a combat showdown, all Deathknell triggers created by lethal combat damage are held as Pending Items until after the units are moved to the trash and surviving units have been healed. The Deathknell effects then resolve in the final step of the combat cleanup process (see RiftJudge FAQ #10444).
• "May" Abilities: Remember that if a Deathknell ability contains "you may," it is optional. The controller decides whether or not to place the trigger on the chain at all when the condition is met.

This is my interpretation based on Rule 808 and the provided FAQs. No official FAQ exists for this exact scenario, but this aligns with the documented FEPR (Finalize, Execute, Pass, Resolve) process for simultaneous triggers.