First of all, this build is NOT for hardcore matches.
This is just a fun build if you are feeling a little trolly with Karthus. Quite a bit of this build will seem to be done by a 5 year old, I know. I don't intend for this to look spectacular and the best thing ever. This was going to be pretty amateur due to me getting into league just a few months ago. If you are ok with all of this, then have a heck of a time.

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Karthus is very reliant on his abilities. They are what compose him. They are what makes him the kill thief that we are going to make him to be.


Q:

Though it has a very fast cooldown and doesn't use all that much mana, I don't use the Q at all due to its very small amounts of damage, and also that we want to get our W and E upgraded as fast as possible.

W:

This ability comes very handy when fleeing or trying to keep someone from fleeing. It slows people down and allows you to get that final hit or make it back to the turret just in time.

E:

E is very useful to get your last hits and is an excellent for of creep clearing due to its fair amounts of magic damage and its large radius of effect. You need to be careful using it early game because it will drain your mana very fast. Mid Game though, when you have larger amounts of mana and faster mana regen, it becomes a very useful ability.

R:

This is our ultimate. This is what gives us our kill stealing power. What you usually want to do when your cooldown on this is done is scout the map and try to find any enemy champions with low health. This is when you go in for the kill. Use your ultimate and take that kill for yourself. It's all yours.

Passive:

Apon death, you will still be able to cast spells. Use this to your advantage and, with your last 7 seconds of life, finish off your killer.

Pros:

Ultimate becomes a powerhouse

Kills from anywhere in the map

Creep Clearing

Crowd Control

Cons:

Lack of a good escape

Teammates will hate you

Helpless without mana

Focusing mainly on cooldown reduction and ability power, your ultimate becomes a powerhouse anf the worst kind of kill thief. Karthus is really good at crowd control but

Once again, this is not the best build. I am an amateur so this might even be the worst. This is only if you want to have fun and i would never recommend this for any ranked matches or hardcore PVP.

In conclusion, if you want to be trolly, try out this build and have a little fun.