How does Assassin mastery works on Karthus ultimate?

How does Assassin mastery (+2% damage to champions when no allied champions are nearby) works on Karthus ultimate? Will I do +2% damage when no allied champions are near ME but are near enemy champions? Or does it only works when no allied champions are near enemy champions either? And does it even works on global ultimates or does the whole map is seen as "being nearby" in case of Karthus ult? Is it good on Karthus or should I just take +% buff duration instead?