I'm going to copy and paste my response on another one of the Karthus threads since I don't think anyone saw it. (That thread was kind of old. (Old as in day old and not very popular.))
"I'm silver II, so take this with a grain of salt, but I think that the preseason made Karthus quite strong. (I also have level 5 Mastery on Karthus, but take that with a grain of salt too since I have level 5 with a lot of champions.) His weak early game was improved slightly, his midgame power spike became much stronger, and his incredible lategame teamfighting potential became even better. Why? Rod of Ages got buffed, and that was generally a core item on him unless he got ahead in lane and decided to rush Rylai's instead. This is partly what made his midgame power spike greater, but the real buff that improved his performance during all parts of the game was the addition of Deathfire Touch. He has an incredibly high uptime for it due to his spammable Lay Waste and his toggleable Defile, and so he can be more of a threat in the early game and be even more ridiculous of a threat lategame. While I'm sad to say it, Karthus needed some nerfs to tone down his insane synergy with Deathfire Touch (and to a much lesser extent Rod of Ages), and I'm with Riot on the nerf. It won't allow Karthus to apply his sustained damage as easily, and this will decrease the incredible duration of Deathfire Touch that Karthus was once able to put out.
Karthus is an unpopular champion, but being unpopular has no bearing on whether a champion needs a nerf or not. He became too strong with the preseason patch, and they nerfed him."
I don't main Karthus, but I play him enough to know how strong Deathfire Touch and Rod of Ages made him. He's been strong for awhile, just underplayed. Why is he underplayed? He doesn't have an interesting kit, his ultimate, while awesome, has an extremely long cooldown and some champions (and Zhonya's) can counter it, his early laning phase is weak, his autoattacks are atrocious, and he is extremely immobile. These facts does not mean that Karthus is weak. Karthus has many strengths to offset his weaknesses. He has an extremely poweful ultimate despite its counters, he has ridiculous damage on Lay Waste during the mid and late game for its low cooldown, Defile provides insane teamfight utility through its massive damage and zoning potential, and Wall of Pain ALSO provides insane teamfight utility as well as ridiculous catching potential (very difficult to maneuver around and slows for a ton if not dashed over). His passive is also extremely strong and gives him a counter to focus the carry strategies in that he can still contribute to a fight for seven seconds after his death.
My point is, Karthus was in a good position at the end of Season Five despite his low playrate. When Season Six came around, Karthus got two relatively large indirect buffs. Though he might not have benefitted as much as Brand, he certainly benefitted, and the power that he gained from those buffs was enough to warrant a nerf. Why nerf his wall and not his damage? Because damage is what Karthus contributes to teamfights. Karthus is a teamfighting champion. He is not a catch champion. His wall let him be a catch champion, and his damage allowed him to be a catch and kill champion. His wall will still provide teamfight utility like it always did, but it's power while Karthus is alone is weaker, and that's what is important. If opponents get half their movespeed back after a little while, they can dodge more of Karthus's Lay Wastes and potentially get out of Defile range. His damage was oppressive, but nerfing it was not the way to go, since his damage was all he really brought to a teamfight besides a slow. He needed his slow nerfed so that he could still participate in teamfights like he always did, but not be able reliably apply his damage as he once did without team assistance.
, easily the worst nerf the game has ever seen.
has 53% ban rate, so they decided to nerf 
