Karthus, Eternal

Karthus applies to your other unit's [Deathknell] if the deaths are simultaneous or that unit is killed before Karthus. He doesn't apply if a separate kill action kills him first.

When one kill action kills Karthus and another unit simultaneously,^370.1.a.2 as with The Ruination, the other unit's [Deathknell] is added before the kill instruction moves Karthus off the board.^429.1.a.1.b His passive ability is therefore still active and makes your [Deathknell] effect trigger an additional time.^365.1 If one effect instead performs separate kill actions, those deaths are sequential.^370.1.a.2 Karthus applies only if the [Deathknell] unit is killed first, because his passive ability is active only while he remains on the board.^365.1 When both units instead have lethal damage during a cleanup, the cleanup kills all lethally damaged units together.^323.4 The other unit's [Deathknell] is added before its source moves to the trash, while Karthus's passive ability is still active.^{808.1.d.2365.1}

No. If both [Deathknell] triggers are legal targets for Not So Fast, one copy can counter only one of them.

When a friendly unit with [Deathknell] dies while Karthus is on the board, his ability causes that [Deathknell] to trigger an additional time. Each trigger is added to the chain as its own independent pending item.^808.1.d.2 Not So Fast targets a single spell or ability, so each copy can counter only one trigger.

This applies only when the [Deathknell] ability chooses one of your friendly units or gear. Otherwise, Not So Fast cannot target either trigger.