When a Deathknell ability triggers that targets a card in a public zone, do you react before or after the controller chooses the target?

Ruling: You react after the controller chooses the target. When a triggered ability requires choosing a target from a public zone (like your trash), that choice is made during finalization before the ability is fully placed on the chain. Once finalized with its target chosen, the game state becomes closed and priority passes, allowing you to react with full knowledge of what was targeted.

Sequence:

• The Deathknell ability triggers and is added as a pending item
• During finalization, the controller chooses whether to place the ability on the chain (if optional) and must choose any required targets
• The ability is placed on the chain as a finalized item, creating a closed state
• Priority passes and opponents can now react, knowing which target was chosen
• After all players pass, the chain resolves

Nuances:

• Cards in public zones (like trash) are valid targets and must be chosen during finalization, not resolution
• Rule 355.17 does not apply here because the choice is specifically outlined as targeting in earlier rules