Please read the entire guide and try it before a downvote

, the items and runes may seem a bit odd but let me explain myself through this guide. Karthus is a very useful champion, he can initiate and die then use his passive to unleash havok, or he can stay back chipping away enemy health with Lay Waste.
NOTE:
This is not a very in depth guide, explaining exactly how to play Karthus isn't exactly easy.
IF you want more information on how to play him then you could read another guide, I made this mainly for the items

at the top it says you will have 320 AP but that is wrong!!!!!
You will have much more, stats above are wrong, some of them are wrong

Pros-
Lots of AP
Amazing DMG
INFITE MANA
His ultimate

Cons-
SQUISHY
long cd on Requiem :(
Not too useful til mid/late game

Other viable options would be:
- if you prefer over flash
- to help you in the laning phase of the game

As on all AP champions i take magic pen. marks, these are the only viable option, then for seals and glyphs I take mana per lvl, you may ask why but I will explain that in the items, and I take magic pen. quints for some exptra magic pen because we will not be getting a Void Staff.
If you want to change the glyphs for some CDR per lvl thats fine, it won't effect your dmg very much and you will be able to use your ult more often

Standard 9/0/21. Taking magic pen. in offensive and taking reducing death time to synergize well with and again, I take the increased max mana, as having a lot of mana will be useful.

I start with a with the crystal for the nice 200 mana, allowing me to use my Lay Waste more often and not worry about running out of mana, as soon as I hav enough for Tear of the Goddess I recall(Asuming enemy mid champ isn't on your turret).
ALWAYS GET TEAR OF THE GODDESS

BEFORE

BOOTS
Youd will increase your max mana very quickly by getting Tear first as opposed to boots,

SO WHY DO WE NEED SO MUCH MANA????

Archangel's passive is that 3% of your mana is added to your ap, and you hav the 155% boost from . This is why I chose many mana runes and the max mana mastery. Having 4 archangel staves will make you hav 12% of your max mana is made into AP. The first archangel staff will give you 1400 mana(400 from item,1000 from unique passive) the other 3 will give 1200 total.
Late game if you want and the other team doesn't have a lot of MR, you can sell ur boots for another archangels giving you around 1000 AP :)
THIS IS THE BEST WAY TO GET MAXIMUM AP
You can get more AP this way instead of buying a lot of rabadons, with all these items you should have around 700-800 AP or so, making ur ult do around 800 dmg :o. In addition you have rabadons to also greatly increase your AP from archangels. I also like this because I never run out of mana...You can keep ur Deflie on all battle and cast Lay Waste as fast as it cools down and still hav a lot of mana leftover.

I like to harass with my Q and then go in close with Deflie and put down a Wall of Pain on the enemy champ and then blast them away with Lay Waste. But in a team fight you should be the first to die anyways, then just worry about helping out while your dead for those 7 seconds. The enemy team wont realize their health dropping by your 800 AP and deflie then your ult.

Comment and Vote on what I can change, this is my first guide I wrote, i figure i'd say this down here so people would at least read this instead of seeing "This is my first guide" and then they don't read it.
Use this and tell me how it goes, its a bit far fetched but it works great
OH