If you're not so fast a Deathknell ability but they have Karthus, does it cancel only one trigger or both?

If you have a unit with a Deathknell ability and you control Karthus, Eternal, the Deathknell will trigger twice (the base trigger + one additional trigger from Karthus).

If an effect "cancels" one of those triggers, it depends on how the cancellation is worded, but generally:

1. If the effect cancels "a Deathknell trigger": It will cancel only one of the two triggers. The other trigger will still resolve normally.
2. If the effect cancels "all Deathknell triggers" or "the Deathknell effect": It would cancel both.

Why this happens:

According to RiftJudge FAQ #8383 and FAQ #494, Karthus's ability is a passive ability that adds an additional instance of the trigger to the chain. Because these are treated as separate trigger events on the chain, a card that targets or cancels "a trigger" will only affect the specific instance it targets.

Key points:

• Passive Ability: Karthus does not "double" the effect; he causes the ability to trigger an additional time.
• Separate Triggers: Each trigger is an individual item on the chain. If you have an effect that says "Counter target triggered ability," you would have to use it twice to stop both triggers.

If you have a specific card in mind that is doing the "canceling," please provide its name so I can confirm exactly how it interacts with the chain!