This is my 1st guide, I'll try my best but dont expect nothing to fancy.

Now getting to what matters:
I played karthus for a long time, tryed many builds and had some experimenting of my own. So after many changes i got a nice and diferent build but very effective.
This build will enable you to take lots of damage before going down and will still maintain you with a nice damage output.

You may thing this build is a bit crazy but give it a try before flaming.

REDs
Greater Mark of Magic Penetration

I think there is no doubt about this one, as karthus is a caster he will need this a LOT.

BLUEs
Greater Glyph of Cooldown Reduction

CD reduction is allways nice so you have your "R" ready to action.

YELLOWs
Greater Seal of Vitality

To be honest you can use other runes here maybe it will be more effective, just i allways used this ones and they do fine.

QUINTESSENCEs
Greater Quintessence of Magic Penetration

Same reason as REDs, and i normaly dont buy void so i really need these.

Ok now the "crazy" part.

I start with Amplifying Tome and a Health Potion and i try to get mejin's soulstealer + lvl1 Boots as soon as i can. Then i buy a Catalyst the Protector for some durability and finish the Sorcerer's Shoes. Then i go for Rabadon's Deathcap for damage and Warmog's Armor after for crazy HP. The last 2 itens depend on how you're doing and the opposite team, if they main dmg is AP or you're in litle truble with their CC finish your Banshee's Veil 1st, if they are a litle more AD heavy or you just need some more damage go for the Zhonya's Hourglass.

You can go for Void Staff insted of Zhonya's Hourglass if they are stacking up too much Mres for your runes to cover it. OR Abyssal Mask if your team can benefit more from it than you with void (i never do abyssal though ... if i need Mpen i go for Void, still it's an option)

Also you can go for Rod of Ages insted of Banshee's Veil if you feel like you need a litle more AP insted of the Mres.

I take Ignite and Ghost

ignite helps you many times to get 1st blood or helps get someone low enough to ulti.

As for ghost is nice so you can maintain people in your E for longer time, helps with chases or running. You can also instead of ghost take Flash, i just prefer ghost becouse of the 1st reason "you can maintain people in your E for longer time".

PROS
- Lots of hp wish makes you not so easy to kill
- Nice damage
- With all stacks "R" have very low CD
- Easy to stack Mejin's becouse of "R"

CONS
- You REALLY need blue with this build ... that or a few mana pots with you
- Try not to die too much as almost half your AP comes from stacks ... lose those and you lose lots of dmg output.

And this is it for my 1st guide.

I though of creating something a bit diferent than usual, hope that you guys enjoy playing with this build as much as i do. I allways like those guys ingame that say OH KARTHUS WITH WARMOG and laugh and then end up losing becouse of it.

Any constructive criticism will be wellcome, if it helps me betting the build better. Random flamming will be just igored though.

oh well i dont know what more to say... hope this "crazy" build is of some help for you guys... have fun.