When Karthus dies and goes into his passive, if he is standing on the ascension platform when the buff descends, he will still receive the buff. He's not reviving like Anivia or Aatrox, dead should mean dead.
Anivia and Aatrox passives are revivals, you have a short window and you are back in the fight With Karthus, once you are dead, you gotta wait the timer out, you get 7 seconds of vengeance but after that you get the grey
This happens with most if not all Undead Passive Champs. It's a feature :P This has always happened during ascension.
