Not Updated For Current Season
Ok, as you can see, this is a headbreaker-build; I mean, this Karthus would be the fear of all the players... his ultimate would be letal.
Of course, this build can be modified depending the battle...
Well, I explain. This karthus is letal, but you have to keep the distance with the enemy champions, because they'll see that you are a big problem, and they'll focus all the aims in killing you.
At the beginning:
If you play a 5v5, go to the mid. It's the best. In that case, you have to play with Q a lot, and when you level up active E to recover your mana, also clarity would help on it. So, it's important that you kill him, but don't obsess with it. When you level up to 6, if you haven't kill him/her yet, it's sure that you will. (If you found a invisible enemy, it's more easy, buy a detector (150coins) and with Q you'll kill him/her quickly, because normally people playing with invisible champions are less careful)
Ok, when the you'll be 11 or more, you can start helping the other champions. It's important to know that Q makes double damage if only hit one target, in that case when you are going to fight try firstly to hit them with W, and then it'll be easier to hit them with Q. If you see that they go to you, scare them with E (but desactive when they leave because it waste your mana!!!). Finally, if they scape it's the time of R, the ultimate. I promise that it's really fun when you kill 3 or more enemies with R. (It's important the fiendish codex and deathfire grasp because they reduce the skill cooldown)
And well, that's all, be careful because karthus it's very weak, try to control all the enemies to know when active the ultimate, and of course get fun!!
Well, I'm not english, I tried to write the build the best I can. I hope it would like you. With this build, well not finished because I'm 20, I finished games with 17.1.10, 15.0.9, 17.2.7 and scores like this. When the games goes wrong, it's your time to change the build, because you would need some defense, Frozen heart and spirit visage could be a good choice!!