When Baited Hook sacrifices a unit with Deathknell and finds Karthus, in what order do effects resolve and does Karthus double the Deathknell trigger?

Ruling: The Deathknell trigger only happens once because Karthus enters the board after the Deathknell trigger has already been created. Karthus needs to be on the board when the trigger happens (not when it resolves) to double it.

Sequence:

• Baited Hook resolves, killing the targeted unit
• The Deathknell trigger becomes pending but cannot enter the chain until Hook finishes resolving
• Hook continues resolving and adds the found unit (Karthus) as a pending item
• Pending items finalize in the order they became pending: first the Deathknell trigger enters the chain, then Karthus enters the chain on top of it
• Karthus resolves immediately as a permanent, entering the board
• Players receive priority to respond to the Deathknell trigger
• The Deathknell trigger resolves

Nuances:

• Effects cannot be added to the chain mid-resolution of another effect
• Karthus must already be on the board when the trigger happens (not just when it resolves) for his doubling effect to apply