Currently tank 
is totally broken, Riot should do to him what they did to Ekko. Here is my idea:
E dmg per tick from 30/50/70/90/110 + (20% AP) to 10/30/50/70/90 + (30% AP)
R dmg from 250 / 400 / 550 (+ 60% AP) to 150/300/ 450 + (75% AP)
....
This is totally not to bait Riot in buffing
Yeah...tank Karthus is totally busted, but these changes don't go too far enough. R damage needs its damage reduced to 100/200/300 (+130% AP). Then no one will build Karthus like a tank ever again, the end.
While were at it lets go ahead and get project: Karthus as well ( ͡° ͜ʖ ͡°)
tank karthus? what? All he has going for him is a slow, Karthus has no mobility. Tank ekko had mobility, a slow, a situational stun, a heal and reposition ult and had good base damage.
Karthus is a scaling mage, I...don't even know who thought this could be a thing
Actually tankarthus doesnt sound bad at all lol think about it, get Rylai's and liandries and full tank, major damage even when u die
what game did you play where this happen
