Good guide :)

Mejai's isn't good on Karthus because he usually dies. Also some more damage from Archangel's should be included and pictures to prove it works.

mejai doesnt work for karthus since ur job is to die in the best place possible. U also NEED Archangel because u can easily stack it, making ur mana infinite and giving u a great amount of AP. Dont use rod of ages, if will make the enemy dont focus u and u want it, let them focus, use ur defile, zhonya, wall of pain to zone them, die and ult to get that pentakill.

Put your pro/cons in the beginning of your guide because it's supposed to act as a quick reference for people to keep in mind as they read on. Also never include Mejai's Soulstealer in your core build because it is ALWAYS situational, and even then I would honestly NEVER recommend it. If you like pub stomping kids on your smurf go ahead, but as soon as someone with moderate skill sees you're building it, you're going to be focused down so fast in any fight or gank. Also, karthus by nature with his Death Defied dies very often as part of the strategy and is arguably supposed to die in teamfights because then he can channel his ulti without risk of cancellation. So you wanna lose a third of your stacks every teamfight? that's no fun. Your runes also have this weird mix of flat and per level AP that I really don't think optimizes your build. You do still need magic penetration early before you get Void Staff so substitute those for your current marks. As is, this guide does not deserve a vote, because of some serious flaws in logic, but if you work on it I'll come back and be happy to help you out.

nice build