Both Karthus and Cassiopeia have been officially disabled, thanks to the AoE bug I mentioned earlier today. As I said in that post, those two champions in particular seem to benefit in big ways from the problem, though Riot wasn’t clear on how widespread the issue actually is.
Here’s the official post:
Due to a current bug affecting AOE damage spells, we are temporarily disabling Karthus and Cassiopeia. These champions are disproportionately affected by this issue and will be re-enabled when we release a hotfix. Rest assured that we have a dedicated team working to resolve the issue as quickly as possible.
This is one of few times that we’ve seen champions disabled thanks to bugs associated with their skills. I’m glad to see it, though. I was getting sick of Karthus hitting me from crazy range.