Does Karthur Eternal trigger the Deathknell effects of other units again when he dies in the same damage instance?
Yes, if Karthus, Eternal dies at the same time as another unit with a Deathknell ability (such as during a board wipe or a single instance of combat damage), that unit's Deathknell effect will still trigger an additional time.
According to RiftJudge FAQ #9317, FAQ #10162, and FAQ #10742, this interaction works because Karthus's ability ("Your [Deathknell] effects trigger an additional time") is a passive ability, not a triggered ability.
Why this works:
- Passive vs. Triggered: Unlike triggered abilities (which must be on the battlefield to evaluate their own trigger conditions, per Rule 383.2.c.2), Karthus's ability is a passive ability. It modifies the game state for as long as he is on the battlefield.
- Simultaneous Events: Because it is a passive ability, it remains active at the exact moment he is killed. It does not need to be "alive" to evaluate whether it should be active; it simply modifies the rules of the game at the moment the other unit's Deathknell trigger is created.
- Result: The other unit’s Deathknell triggers once for its own death, plus one additional time due to Karthus's passive ability.
This is distinct from other abilities (like those of Viktor, Leader) which fail to trigger if the source leaves the board at the same time the trigger condition is met, as those abilities must evaluate their own trigger condition at that specific moment.