Karthus - Buff

Passive- is fine

Q is fine

W . is fine

E I think that the damage should be lowered and have an added effect where the longer they stay in his E the AOE aura thing it should increase in damage over time and increase his damage of his other skills for a % of how long the enemy has been in his Defile because it makes them filthy.

R- his Ult he should be able to Ult every enemy minion. so he would be more useful and have more wave clear.