Already entered a comment I wish not to repeat, thank you very much ^^

I don't see what is wrong with this build but here is my opinion. Karthus does like to go in head-strong but he is ultimately effective at melee champs because of his defile ability and with sufficient AP can deal tons to a dps/burst champ like akali or yi before they reach u. However, mejais is good and bad. On the account of that during mid-late game, Karthus likes to go in first and deal as much damage as he can with defile and lay waste. (also to slow down enemies when needed). And then his ulti just mops up everything (and if someones gets away, your team would be able to chase them down). So mejais is optional, you could replace that with another archangels. And another suggestion, the abyssal scepter is a good item however, i would go with zhonyas hourglass for the main reason that when a Karthus player is doing well, most players would attempt to take down Karthus first before he can deal too much damage. And with the hourglass, its active ability is so effective that enemy players walk into their doom. Basically you could run into a 5v5 battle, click defile and activated the hourglass. You remain untargetable and you deal lots of damage. As i said these are just suggestions to how I play, i dont have a build on mobafire but its what i think of Karthus. Flash is good as well, and i like how u included Tear of the Goddess for mana but with players who have mana regen runes, its much harder for Karthus to be oom. Anyway, its a decent build - dont take it wrong, its not great either but you still get a +1 from me and some suggestions towards tweaks ^^

hmm, don't really understand why people say it doesn't have a lot of AP, end game with this build i am above 800 ap with a decent amount of magic pen, don't really see how you can get a lot more AP without stacking archangel's.

to shin: i understand what your frined say's but, the passive is quite easy to avoid unless the enemy team are complete noobs. and i don't like to give kills away.

When i play my build i end with 15-20/0-2/10-20 almost all the time. (unless the game end's fast )

to aplha, please don't just downrate to downrate, your build os almost the same except you go rod of ages instead of abyssal scepter, and tbh 30 AP instead of 20 magic pen for your entire team, i prefer the magic pen.

its alright

Not a very good looking build to be honest and I despise Karthus. I just don't see how it is fair for a champion to be able to do what he does. But putting that aside, I really don't think Mejai's Soulstealer is a good item on Karthus. I have a friend who basically always plays Karthus and he is certain that suicide Karthus is the way to go. His passive allows him to deal insane damage around him for 8 seconds and his ultimate mops them up. Also their isn't much Ability Power. Karthus is a glass cannon and so he should be built like one. I don't see this build allowing Karthus to operate to his full potential.

i didn't like it. it failed hard core

I don't see enough AP here.