If you have 2 Karthus on board and a Deathknell unit dies, do you get the Deathknell effect 3 times or 4 times?

Ruling: You get the effect 3 times total. Each Karthus triggers based on the initial Deathknell trigger, not on each other's added triggers.

Sequence:

• Deathknell unit dies, triggering its Deathknell effect once
• First Karthus sees the Deathknell trigger and adds one additional trigger
• Second Karthus sees the Deathknell trigger and adds one additional trigger
• Total: 3 triggers (1 original + 1 from first Karthus + 1 from second Karthus)

Nuances:

• Karthus is a passive effect that looks for Deathknell triggers being finalized, not when the effect itself resolves
• Each Karthus does not see or respond to the additional triggers created by other Karthus copies