When do Deathknell effects trigger? For example, do we play the Deathknell unit in the trash first and then the effect triggers, or does it trigger while the unit is on the board but about to die?

Deathknell effects trigger when the unit is in the process of being killed and sent to the trash. According to Rule 808.1.d, the trigger is the unit being killed and sent to the trash.

To reconcile the apparent conflict between the provided FAQs, it is important to distinguish between triggering (the event) and resolving (when the effect actually happens).

The Triggering Process

When a unit with Deathknell is killed (by a spell, ability, or combat cleanup), the game marks the unit for death and creates the Deathknell trigger. At this moment, the Deathknell trigger is generated as a Pending Item on the chain (Rule 808.1.d.2). The game notes the unit's attributes and location before it leaves the board (Rule 808.1.d.3).

The Resolution Timing

The confusion regarding "immediate" versus "pending" resolution is resolved by the game's FEPR (Finalize, Execute, Pass, Resolve) process:

1. Pending State: The Deathknell trigger is created as a "Pending Item" the moment the unit is killed.
2. Finalization: A Pending Item cannot be resolved until it is "finalized" and placed onto the chain as a standard, active chain item.
3. Timing:
   • During Chain Resolution (e.g., a spell kills the unit): Per FAQ #3119, when a unit dies during the resolution of a spell, the Deathknell trigger is finalized and added to the chain before the rest of the existing chain resolves. Because the new Deathknell trigger is added to the top of the chain (LIFO), it effectively interrupts the previous stack, resolving before the remaining items that were already higher on the chain than the spell that triggered the death.
   • During Combat Cleanup: Per FAQ #10444, combat is a distinct process. Deathknell triggers resulting from combat damage are added as pending items, but they are specifically set to resolve after combat healing and recall have occurred, during the combat cleanup's resolution step.

Summary

• Does it trigger while on the board? Yes. The trigger condition (the event of the unit being killed) occurs while the unit is technically still on the board or in the process of being removed from it.
• Does the unit go to the trash first? The unit is moved to the trash as part of the kill action. The Deathknell trigger is recorded just before this movement (Rule 808.1.d.2) to ensure the game has the necessary information to resolve the effect later.
• Is it "immediate"? In the context of a spell resolution (FAQ #3119), it is immediate in that it forces the Deathknell effect to resolve before the rest of the chain continues. It does not wait for the entire chain to clear, but it does wait for the game to complete the current FEPR finalization step to "finalize" the pending trigger.