First I would like to start off by saying hello and thank you for taking the time to view my guide. I main as karthus and I am pretty successful with him. I go around 20+/-10/10+ a game.

For the item build I like to start off like so:

First buy: Mana Crystal + 2 Health Pots.
Second buy: Tear of the Goddess, Boots, and Amplifying Tome.
Third buy: Sorc Boots + Mejais Soulstealer
Fourth buy: Needlessly Large Rod
Fifth buy: Zhonyas Ring
Sixth buy: Archangel Staff
Seventh buy: Guardians Angel
Final buy: Banshees Veil

If you can't afford a main item than buy the recipes. It will help!
If you want 1.2k AP than just stack 5 archangel staffs and 1 zhonyas ring.

When you play karthus, you wan't to mid EVERY TIME. Your ult is one of your best spells...
If you can't mid because random people that you solo que with are stupid..than suck it up and lane...but I personally prefer MID.

In mid lane, at level 1 you don't want to necessarily spam Q[Lay Waste] because it will just drain your mana and get you killed.

At level one, Your first spell should be Q[Lay Waste]. I normally don't use this until level 2. Reason being is i don't get anything in return for it. At level 2 you want to get your E ability. It returns some mana and is very helpful. At level 3 you should be able to dominate the enemy mid champion, putting your 3rd point into Q. My summoner spells are usually Ignite and Flash. At level 3, I cast my E ability, Flash to the enemy, ignite, than spam Lay waste. Skillshots are key to success. Don't be afraid to chase, you want him in your circle. Try to lead your shots with Q. Think ahead of them...where would they run..remember, your Q does DOUBLE THE DAMAGE to single targets..

1-9 you switch between Q and E, Q being your main priority skill. At level 10 i put a point into W [wall of Pain] and either continue pushing mid or help my team with ganks. Your ult at level 1 does 250 damage. I usually never use my ult if the enemy is over 200 health. I ult to kill. If i see a team mate being chased, i ULT and ping the enemy chasing him several times...so my team mate knows to turn around and attack...This means you get a assist and your team mate gets a kill.

DO NOT be afraid to die with karthus. Your passive is amazing...I am usually the first one to get into battle because i am cocky, greedy, and cocky... I hit my e, wall the enemies, and flash in spamming q at every target.. getting almost the whole enemy team extremely low or dead...sometimes i even survive. IF i die, i spam q for a few seconds than ult, ensuring a team battle victory! And hey...if you do die..20% reduced dead time = WIN.

this is just a quick guide to karthus and should work...if it doesnt, play Garen and buy sunfire capes.

Kthx..Bai.

P.S - Thanks to Fat Girl Rodeo for getting me into karthus!!