User blog comment:Atmey/Aghanim's Scepter/@comment-5416973-20120910211239/@comment-5416973-20120911051847

Yah, but since Karthus gains "an 450 AoE damage centered around the targets but does damage the main target, the AoE magic damage is 60% of the main damage." That means if Galio taunts the entire enemy team, and they are within 450 range of each other then karthus will do 300% extra damage on top of his original 100%, since 60*5=300.