Thanks you proved my point you know nothing about playing Karthus and can't even answer a simple queston plus his Q already dose double damage do you know how much that is oh wait you don't because you play mostly adc champs top lane and don't play mid or even Karthus you you wouldn't know how mutch damage tripling a single target Q would even do.
You don't even know how much damage a single q dose and what items are needed to deal damage with a q.
You as a proud elo player with no real knowledge of this champ or his play stile is calling my idea ridiculous I will have you know I suggested the change for soraka heal to use her own health a month be for it hit pbe and I also predicted that dfg spell vamp was going to get removed and rage blade was going to be supper strong
Yah how embarrassing it is that I have such a good understanding of a game that a high elo player doesn't.
SO again how mutch damage would a triple damage Q do with 80 ap it really simple. Ho and do you think you as a gold player couldn't doge it.
What is the dps of his Q at level 10 to the aa of Riven that is an easy one. Kathus only has one itam by ten that gives ap
or 
So level 10 Riven dps and don't forget that 25% extra damage from your passive vs Karthus dps so me your elo knowledge. Plus Riven can still do crit damage so again his dps damage comparison.