If you Ruination and your opponent has Karthus, are all Deathknells still doubled?

Yes, all Deathknell effects will still trigger twice (the base trigger plus one additional trigger from Karthus).

According to multiple RiftJudge FAQs (including #9028, #9711, and #10162), this interaction occurs because Karthus, Eternal's ability ("Your [Deathknell] effects trigger an additional time") is a passive ability, not a triggered ability.

Why this happens:

1. Passive vs. Triggered: Because Karthus's ability is passive, it does not need to evaluate its own trigger condition at the moment of death. It simply applies its effect to the game state as long as Karthus is on the board.
2. Simultaneous Events: When Karthus and other units with Deathknell are killed simultaneously (such as from a board wipe like Ruination), Karthus's passive effect is still considered active at the exact moment the other units' Deathknell triggers are created.
3. Result: Each unit with a Deathknell effect will trigger once for its own death, plus one additional time due to Karthus's passive ability.

This is distinct from triggered abilities (which would fail to trigger if their source was removed from the board at the same time the trigger condition was met); because Karthus provides a passive modifier, he successfully doubles the Deathknell triggers even while he is being killed by the same effect.