Build Guide by Lastobelisk
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Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build is best used in mid, buy a regrowth pendant bully your lane until you have 365 gold, recall and buy your philosopher stone then port back. This will allow you to stay in your lane longer and will help get gold. Around the time you will be reaching level 6 you should have enough money for your Maji's soul stealer and your port should be back up assuming you used it right when you reached the first 365 gold. Grab the item and port back to your tower and wait for the right time to use your ultimate.
Karthus is soft so you want to stay near the tower as much as possible if you push them against their tower or kill them take that time to go grab the blue buff to reduce the cool-down on your ultimate. You want to play it safe at first until you have all your items then you will be able to run people over.
The first skill you should get and max out is Lay Waste it is the best bully skill you have and in combo with your other skills will wreck the enemy.
Secondly get Wall of Pain not only does it work well to keep enemies at bay but it also reduces the enemies magic resistance which will aid you in killing them faster and easier.
After that Rotate getting skills between Lay Waste and Defile while still getting your ultimate every time it is available, save the last 4 levels of Wall of Pain for last while it is handy it is least beneficial until the end game.
In the end game if played correctly you should be able to enter a team fight by throwing down a Wall of Pain on the enemies, turning on Defile and going in nuking away with Lay Waste. *** Note*** use Requiem after death so it doesn't get interrupted.
Once you have all your items sell the Philosopher Stone and buy a Void Staff.
This is the build that I use and when I play Karthus i win 90% of my games, it may take a little bit to get the timing down but once you do it is easy.
When you go to get your philosopher stone you should just be running out of mana w/o the use of clarity so when you port back you should have full mana and clarity available which should be enough to last you until level 6 when you can get back and get your Soulstealer at this time your port should be back up if not wait for it. After that everything should fall into place...if you are keeping the enemy in your lane pushed back take time to kill the blue buff and jungle mobs this will give you that much more of an advantage. On average I take 1/3 of all my enemies life when I use my ultimate unless they are using Banshess Veil..
***Note*** The Skill Sequence may be slightly incorrect I don't know the levels I just know in what order I pick them Requiem>Lay Waste>Defile>Wall of Pain
Once you learn how to kill people without the use of your Ultimate then you will be unstoppable.