If I have two Mixologists with Temporary in base, and they die. At the beginning phase of the next turn the first one bringing back Karthus, would Karthus enable the second Mixologist to bring back 2 units instead of 1?
No, the second Deathknell trigger will not trigger twice.
According to RiftJudge FAQ #9606 and FAQ #10300, the number of times a Deathknell ability triggers is determined at the moment of death, based on the board state at that exact time.
Why this happens:
- Snapshot of State: When your two Glasc Mixologist units die, the game checks the board state to determine how many times their Deathknell triggers will resolve. Since Karthus, Eternal was not on the battlefield at that moment, he does not modify these triggers.
- Locking the Chain: Once the two Deathknell triggers are added to the chain as pending items, the total number of resolutions for each trigger is fixed.
- No Retroactive Modification: When you resolve the first Deathknell trigger and play Karthus, Eternal from your trash, he enters the battlefield. However, he is too late to modify the second Deathknell trigger that was already placed on the chain before he entered play.
In summary, Karthus, Eternal must be present on the board at the moment a unit with Deathknell dies in order for his passive ability to increase the number of triggers for that specific event.