Karthus is pretty balanced
As long as champions with spammable dashes are op like irelia, karthus is playing with 2 abilities against them. This is why he has an average win rate in master+.
However there is a strategy to actually make him overpowered. Karthus smite mid is beyond busted when played properly and will dominate solo queue when people realize how broken it is. You will hit level 6 when everybody else is level 4. Runic echoes is a good item as it is a cheaper version of ludens and gives you a combat summoner spell. You were going to play farming simulator anyways and this is the best way to do it.
