Hello, i'm no pro, but i have been using Karthus quite a while. About 2 weeks ;p I only started the game about a month. This is my first guide. Okay, back to the topic, Kill stealing(joking Karthus has obviously been mistaken as a kill stealing champion) Only noobs think it kill steals, go look at those ranked game, ppl ask him to kill the opponent, cos they know what the ultimate goal is winning, instead of just individual goals, those noobs blame so dun care, but be prepared. trolololo ;p it has been pretty fun. But honestly Karthus is no easy champion, though it seems so. Those who never play with before just say press R press R.

Pros-

1. High damage output
2. Good to kill steal people
3. Can slow
4. Can reduce magic resist and armour
5. Strong AOE
6. GOOD in teamfight
7. STRONG PASSIVE
Cons-

1. SUPER SQUISHY
2. If not farm well, suck!
3. Hard to master ( Most skills is skill shot based)

Runes are mainly to aid in early game. So i built on Magic Penetration and Ability Power to harass/kill my opponent easily. Magic penetration will help a lot in destroying the magic resist of opponents and ability power to increase the damage big time. From other guide, some of them build of health to increase survibility as karthus alr hav good ap etc etc.

Masteries will also help in early game. As you can see that I have 21 points in utility and 9 points in attack. Why. Because i feel that offensively, karthus is good enough, but it is short off mana regen and cool down in early game. So it will bring some problems to farm. So i really put mana regen. Cool down can also help the cool down of requeim. c:

My items as you can see is real different from most people. I like to damage a lot, but not really defend. But of cos i want to survive longer to kill more. So i buy will of ancients to survive especially in teamfights when i use defile, i hardly die, cos every seconds, i gain huge amount of health. Archangel staff is really important for this build, why, cos of the mana it gives, helping you to spam skill especially defile. So i always start with meki peendant, to aid in early game to let me spam lay waste and once i reach the right amount i recall to get mine tear of goddess and continue my build. Rabadon of cos is for its passive. If you use a lot of spells, archangel is real useful. Cos it has a higher ap than rabandon. Rabadon and archangel is a real good combo.

Okay, now skill.
Lay waste- this skill is good, especially when it doubled up when hit only one opponent. And this skill needs a lot of practice into it. So practice more and it will work.
Wall of pain- This skills in one of the best skills in the game. It can slow anything down, lower magic resist and armor down, so your damage will be more strong and help in teamfight. And it have a wide range. But it needs skills to place it at the right spot. And it can help you to escape or chase.
Defile- Early game, use this to farm only on a lot of low health creeps, don't use it too often, cos you will lose mana and be pretty useless. And it doesn't give you much damage early game. But the good thing is the passive it has, you gain mana for every creep or champ you kill which is pretty important. Once your mana get higher you can use it to kick ***! And cos of the range you can use to chase.
Requiem- This skill requires map awareness and some skill. As you need to know the magic resist and health of opponent. Sometimes this skill is only useful if you have good teammates, to tell you someone is low on health.

Normally, i will on wall of pain of my opponent or its escape route, then on my defile. ( that pretty does it ;p) just joking. Of cos if you use lay waste it will be stronger, if they manage to successfully run away use requiem. But if you are really a noob, just spam

Okay, I use flash and teleport. why? cos flash is to help me run away or chase and teleport as i recall quite often and help me to gank. simple. ;p
Erm.. i do use heal last time, cos to survive. But only works in early game. The other summoners spells, i'm to lazy to use, as flash and teleport is working fine for me. For those like ignite this kind of skills, or exhaust ghost, they are also good. ignite works for killing them if they manage to run away, but requiem is there to save the world! HAHAS exhaust is to slow them down , but wall of pain is also there. for ghost, i won't argue much, its good! hahas summarising karthus have a super good skillset which have whatever a champion pretty needs, except he is slow and squishy . but everything have weakness right? hahas

LAST HIT! LAST HIT! it is very important especially Karthus, only with good farming, you get good karthus! understand! Ohya karthus is quite dependent in blue buff until your archangel staff become imba! hahas

I know this build is super wordy, sorry, i dunno how to use the codes and all that. once i learn more, and get better at karthus i will certainly edit this build and add more pics. lets say karthus is a good champ tat needs alot of practice, use my advice i'm sure you will get at least average 10 kills per game. TROLOLOLOL! c: thks and vote up!