Quite good build but I think that there is Flash missing here. Overmore, I think that it's not good to buy Archangel's Staff as the fifth item because it almost never charges. I can't upvote because of this reasons but I can't even downvote because this build is good in all other aspects.

Thanks for the comments!

Itasko: If you have not tried building items in the order I have listed, test it out, you might find it to work as well as I have. I get the Tear of the Goddess early to begin gaining mana. It is extremely rare that I run out of mana. I just feel that the AP gain from Rabadon's Deathcap and the Magic Pen from Void Staff should come before a complete AA. Plus, if you have Rabadon's already when you buy AA, AA will be that much better. The only time I have early game survivabiliy problems is when I am facing one of my HATE champions, the Magic Vamp usually keeps me from having surviving issues 1v1 and usually 2v1 as well.

I don't like the order of your build and lack of early game survivability. I feel a RoA is more viable and should be rushed early game for health, mana, and AP. Then rush AA staff. But you have AA staff last, which Karthus needs mana ASAP. I do like the items in your build, just not the order. Very well explained though. I will upvote, but please consider some early game survivability. And I would suggest putting AA staff much earlier, because Karthus is very mana intensive, even if you are mana smart.

BTW, where is everyone getting their cool sigs? I want one xD

+1 for the unusual idea of spell vamp. Ill be sure to try it. Most of the time i spend too much on pots.

Perhaps you could have explained the 2 different builds, even a sentence or two to say how the second build is only when your team is doing well, or the first is the standard build etc. Other than this, the build is very well explained. +1

I really like the idea of getting spell vamp on karthus, but guide can use more organization.

+1 to start you off.