Karthus is a very unique champion in the fact that really, you WANT to die in the team fight; preferably with the entire enemy team around you. He is a good farmer with Lay Waste, he has an amazing slow with Wall of Pain, and good AOE damage from Defile. But Requiem is mainly what I want to talk about. Karthus’ Ultimate is one of the most annoying in the game. It has high base damage with a 0.7 AP ratio, and can and will hit every enemy champion in the game no matter where they are or even if Karthus can see them.
Because there is absolutely no targeting involved, some people (me included) refer to Karthus as a faceroll champ (meaning, you could slam your head on your keyboard somewhere around the 1 key, and roll your face across the keyboard to the \ key) and get kills. I will not deny that playing Karthus affectively excluding his ult does take some skill. But you could theoretically get to level 6, and then just sit in the fountain spamming the R key and get kills. (Hmmm... Now I want to try this…)
Getting magic resist with a Karthus on the enemy team is a must, though I would not say that Karthus is OP. But maybe a small tweak to his Ult would be good. Really it wouldn’t have to be anything drastic. Make it so it would only hit champs that Karthus had sight of when it finishes channeling (still ignoring distance) or make it so it deals less damage the further away Karthus is. And I only say this as a “well that would be nice” scenario, because I do not believe Karthus is over powered. He is just really annoying.
Knowing how to counter him is really the key though. It’s a bunch of simple things like:
1. Don’t bunch up in team fights. (Why would you do this anyway???) His dying inside your team will eat your HP for breakfast with Defile.
2. I don’t care how much damage Karthus can output, DON’T TARGET HIM FIRST! Once he is dead, he will continue living for 8 seconds (usual length of a teamfight). Take out someone else, because then it becomes a 4v5 instead of staying a 5v5. (AND on top of that, Karthus gets to cast at no mana cost once he is dead.)
3. Some people come with built in spell shields (Morgana, Sivir, Nocturne). If you are playing one of these champs, you MAY want to save your shield for his ult if you can.