Pros
Crazy damage output
very good passive for team fights
very good farmer
nice skins
global ulti
one of the best teasers
very good slow untibuff

Cons

Very low life
easy target for chams with stelath
high cooldown ulti
no escape

Hello my summoner name is Infamous95 and i wanted to make a different build for karthus. Because karthus is a very powerful character i wanted to make him a build that it will help karthus to do what he is best at killing enemies. I have seen almost all other builds for karthus well most of them are referring to more ranked game karthus with a lot of life and defensive stats. But in normal games you don’t need so much health armor and magic resist but mostly ap and that because of karthus passive. So I hope you will like my build and hopefully you will vote it as a good build. Also I wanted to add that I have used that built about 50-60 times 95% of them it worked.

This runes that I prefer is for maximize dmg at early game and to be sure that the winner of the lane will be you. Also you could use magic pen marks cool down reduction glyphs ap per lvl seals and flat ap Quintessences.This runs might be also good for this build.

9/0/21 I the classical mage masteries the main reason to use this masteries are that you will be more affective to mid and other lanes so I recommend them not only for karhus but almost for every single mage at lol.

Start with dorans ring cause of the health and ap that gives you, because if you don’t have above 30 ap at lvl 1 with karthus you dmg will be almost nothing. I don’t believe that nana is needed for first item cause at lvl 2 you have the passive of the third skill.
for second item i prefer to take (rabadons deathcap) for maximizing the dmg output at early game mostly with ulty and lay waste.
Then i prefer magic pen boots.
Next you should take archangels staff witch is a must for karthus.
We move on to rod of ages mostly for life because at late game you will be the target of everyone.
Then you should take void staff for the magic pen that gives you and this item combined with magic pen boots you will have a lot of magic pen for tanks.
At last I replace dorans ring with Zhonyas hourglass because its passive is combines perfectly with the third attack in large team fights at late game and also it gives you armor and ap.

You should prefer this skill sequence because it max the damage output and combos that you could perform with karthus. Except that it makes easier farming and helps you at all time in the game(early, mid, late game).You could another sequence but change it after six lvl because the sequence until six must be the way I put it.

I prefer those 2 (heal, clarity)for early game at mid lane. You could also use flash and heal or flash and ignite but for early and mid game heal and clarity are the beast. The other two combos are more offensive and you could also try teleport with revive for not loosing any kills but the best is the first combo heal and clarity.

karthus must always until 11 lvl stay at mid. And heal and clarity are ideal for that. When you reach lvl 11 you should go base and take rabadons death cup and if you have more money the magic pen boots. After that you start ganking with team mates. Because ulti has a very long cool down until lvl 16 use it at right times.At lvl 16 and after when big team fights are starting be sure that in the team fight you will have always opened the third skill until you day and your passive is done. When you enter a team fight always use first the second skill at the enemy team for reducing its armor and magic pen and don’t forget that do always ulti when you are at the passive state(when you are dead and your passive has take action)not when you are alive. And be sure that you spam lay waste all the time in team fights and at the whole game.