Sure, lets give it a try. I could tell you beforehand, that your 900 AP will deal more dmg to me, as I dont have mentionable MR... but you would have figured that out, if you would have read the guide ;).

Have fun with your 900 AP, facing the first enemie with MR equipment anyways... your real dmg will be way, way, lower that indicated ingame.

This build is lame . you wanna test your 500 ap vs mine 900? i can do that.

Dont worry Dirty, I already got you right, and I am grateful for your input.
But it would be neither professinal nor fair, if I posted an AP/MP build and telling folks how great it works in comparsion to AP-only, without showing the possibilities an AP-only build would give you as well.

Beside that: MP / MR are quite abstract, if you never dived into the calculations yourself. Therefore I hope, that some hard facts and some fancy colors will ease up its understanding.

I didn't mean it in a way of criticising your build, but rather in a way of complimenting it, as you do a very nice job of balancing magic pen and AP, while most other builds just completely ignore magic pen, which is a huge mistake in my opinion.

Okay, I just wrote an essay, but this editor wouldnt post but rather delete it. WAH!

So once again, but shortened:
As you can see from this Calculation Sheet Screenshot I did some calculations, and compared an AP build (right) to an MP build (left).

As long as your enemie heroes dont reach 85 MR, the AP build is more effective, in terms of the dmg of lay waste & defile.
So in case you play against skilled opponents, that might buy MR... you will most probably not stick to a pure AP build.
Anyway, if they dont get MR and you want to play pure AP, you should get yourselves:
rod of ages, 4x archangels staff, zhonyas ring. This will provide you with another +565 AP.

But in order to play that build, you will:
- have to invest 2.000 gold extra
- have to have -200 HP
- have to get along with 310 movespeed instead of 380 (434 instead of 532 with ghost)
- but you'll have 1.200 extra mana - yeah, tempting I know.

Anyway, there are two things you have to keep in mind:
1st) This values are only applicable to dmg you deal after you hit your target with your wall of pain. Otherwise already 75 MR will even out the advantage (of 565 AP!), and your ulti will be weaker at 75 MR anyways (as you wont place a *** over your targets before you cast requiem).
2nd) Even if a +30% dmg due to more AP sounds tempting to play an AP build: Remember, taht your *** has quite a high CD. You wont be able to profit from the high bonus AP gives you on lay waste & defile while your *** is on cool down. So AP is only granting you a real bonus for ganking mid lvl players. Against better players and for farming, the MP build will be more effective.

Wow nice build. Magic Pen is better on Karthus than AP I think since he has pretty bad ratios and you need as much damage from his ult as possible.

Dirty: Thx for pointing me on this. I am not yet very happy with the solution, but I guess the new pic in the description will at least do it's duty.

The skills are still messed up in the picture in your description.

True, Juffalo, my native language is German - you might have figuered that out by my grammar already. And as it's 7:30AM, my English might have been worse than usual. And thanks for your feedback.

In case somebody should come across problems with understanding my points, feel free to ask me. In worst case, I'll provide a language make-over these days.

I can tell English is not your first language, but I had no problem understanding all your points and a good guide nonetheless.