This is a very nice and to the point guide, I like it. The only thing I have to point out is that the Morello's Evil Tome is mainly exploited for CDR and you don't need any of that as Karthus. I would replace it for a Zhonya's Hourglass.

O and definitely turn on "Comment required for Voting"... that is why you have such a low score.

@ Guidemaker,

Disagree with what your saying completely zhonya's is not necessary because what if your playing against a high AP team??? Then what? It is situational i am only using sorc shoes and rod of ages because they provide magic pen and some mana as well as ability power. Once again morello's is good for your ultimate and again banshee's veil is actually situational again. Please read what i said about when you should use banshee's, QSS etc and then start critising.

@ Awesome Pwnage1,

I believe that getting Rod of ages first is for some survivability archangels is next because of the mana and abiltiy power. Plus rod of ages is a core item. Also thank you for your +1.

I don't like that you get the Rod of Ages before you get Archangel's. The max mana boost and AP is perfect for Karthus and it is better to get it earlier for the sake of a better late-game. I don't like Morello's on Karthus either; you can go with a snowball item like Mejai's or maybe stack some durability depending on whats what. I would also consider getting Wall of Pain at 4 instead of 3. Your Q won't be as useless that way.

P.S. I don't know why people are rating this down, it's actually pretty legit in my view. +1

Rylai's, Zhonya's, and another defensive item should be core. Rod and Rabadon's is good, but Morello's should be kicked out for a Banshee's.