Well this is my first build and i try to make it good...

I will describe how i play Karthus and i named this build like this cause when i am mid the enemy mid will leave in 25% of the games around minute 15.

Teleport is a must.

I recommend Flash but you can also take Clarity.

Greater Mark of Magic Penetration for dmg
Greater Seal Clarity for Manareg(you wont need it late game but its sooo much important early game)
Greater Glyph of Cooldown Reduction
OR
Greater Glyph of Scaling Cooldown Reduction depends what you have already bought and you like more

Greater Quintessence of Magic Penetration for mooaar dmg

Its very important to finish the tear of goddes AS FAST AS POSSIBLE!!!
you wont need sorcerer boots that early cause when you stay mid leveling what you actually should
your Wall of Pain will give you enough time to run into tower range when you got ganked/attacked.

That elexir of brilliance fills the gap till you got your first ap item

Rod of Ages gives you survivability and Archangel staff also scales very well with it.

Archangels's Staff is your first real ap item. With Archangel's Staff and Rod of Ages you got enough mana to use Defile

Now you will get the key item of this build:

After every Lay Waste it will activate and your next auto attack will deal 100% of your ap!
in late game this is a total amount of 800 dmg!!!

Rabadon's Deathcap cause your a caster... nuf said.

Feel free to swap that Zhonyas's hourglass with a Rylai's Crystal Scepter if you need it.

Early Game
Farming early game only with Lay Waste!!! Defile is like manawaste cause dmg is low early game and your mana too.
Keep in mind that every minion kill gives you mana.
Try to get level 6 as fast as possible and LET YOUR TEAM KNOW WHEN YOU REACH!
let your team CALL FOR YOUR ULT!

Try to harras your enemy in mid with Lay Waste when he has half of his health or less left and if you have enough mana, try to kill him
---> run towards him ---> he will panic ---> Wall of Pain + Defile ---> cast some Lay Waste when he runs
he will be dead now or he has to recall
push tower and farm/level
hold in mind that some chars(tris,le blanc, ect) can jump/dash/port through your wall without getting slowed

Mid Game

blabla dont overextend blabla dont run around alone ...you should know that.
Before teamfights try to harras and try to catch some with your wall
your team will move forward then and kill them
in teamfight walk around with Defile activated and spam Lay Waste
if you die try to die in the mid of the fight cause of your passive( Flash can be helpful here) you have infinity mana when your dead so make sure to run defile then!!
make sure to cast your ult then cause the damage burst helps your team a lot and you may kill somebody!!!
in 1v1 always (with Defile activated) cast Lay Waste --> auto attack with Lich Bane active.... repeat that till dead
if they run use your Wall of Pain or requim (in late game often waste wait for a teamfight where its more usefull)

you will die alot in lategame but you will make very many kills/assists cause of your insane dmg output
in lategame use Defile for farming
get blue golem if you have some time

Thanks for reading
pls dont -1 without comment

you can add me in LoL im always searching for good team partners for ranked/unranked matches(**** solo queue)

Eberwurz