@Meddler, Morello, Riot: Can we discuss Karthus?
Karthus isn't in a very good spot right now. Keep in mind he was a very competitive pick back in seasons 1 and 2. Unfortunately, with the shift in season 3 towards assassins he not only was pushed out of the mid lane but he was also given undeserved nerfs for his apparently obscene global presence.
Now in season 4, with assassins having been toned down a bit, the competitive mage picks for mid are those that can burst down their target as fast as possible or clear waves almost instantly. While the latter is somewhat applicable to karthus, he can only clear fast with defile which isn't particularly useful while being sieged because it puts him at risk.
Although the nerf to wall of pain was very detrimental to his usefulness, the issue I have a problem with the most is the cooldown on his ultimate. I can understand that you want to keep a global mechanic in check by not being constantly available due to the overwhelming pressure it generates early game to the lanes. But when you don't have your ultimate back up after a previous team fight when everyone already has theirs back, then it becomes an issue. Especially because late game its global characteristic is lessened significantly when people are grouped up for fights anyway.
The long down time on karth's ult is accentuated by 2 things:
-Karthus normally doesnt build any cdr from items relying on blue buff for a bit of cdr.
-When he does have the cdr from blue buff, it doesn't apply to his ult since most of the time it's used when he's dead so he loses the buff.
Every other ap mid gets to benefit from appropriate cdr itemization and blue buff on their ultimate, but karth doesn't. There are several offenders who have high damaging ultimates with lots of utility with half the cd of karth's. Gragas and annie are the first that come to mind, but there are several others. As experienced as I am using karthus, I've found myself in a handful of situations where a team fight breaks out and my ultimate is on cooldown essentially lacking damage output compared to the enemy mage. What are the balance team's thoughts on Karthus atm?
TLDR: As an avid karthus player, I know how burdensome the global impact from his ult is early game when there isn't much the laners can do about it except "not get low". But late game I see no reason for it to have such a ridiculous cooldown especially when cdr itemization is subpar on him. He needs to have a shorter ult cd at later levels to remain valuable for his team. What are the balance team's thoughts on Karthus atm?
or 
for CDR. Build tanky with 
. That should fix some CDR issues and give you good sustain, as well as good ap from DFG and Zhonyas, added to the slow from Rylais. Also note that CDR is a percentage of cooldowns so a 120 second cd will get 24 seconds off when you have 20% CDR and 48 seconds off when you have the max 40, which would come from runes/masteries, DFG and blue buff. If you're having trouble using your ult in teamfights try to ask yourself if you really need it or if you think your team can pick enemies off without your ult. The emphasis behind's Karthus ult is good timing and strategic thinking. You can't ult willy nilly and expect to turn the tide of a teamfight. That's just not how Karthus works. Also, Karthus top is fairly viable right now. Just that he got pushed out of mid by champs like Katarina, Leblanc, Zed, and Yasuo doesn't mean he's not a good champion.
