WTF THIS IS EXACTLY MY BUILD :DDD AND I JUST DISCOVERED YOURS NOW :D MY BUILD MY BUILD YEEE

i like your guide, but, i don't believe on that build.
Karthus must be able to stand on a team fight so he can do as much dmg as possible and he cant do it without armor and resistance.
Try this one:
Archangel's Staff
Rod of ages
Sorcerer's boots
Zhony's HourGlass
Abyssal Scepter
Rabadon Deathcap

Karthus benefits alot with Zhonya's Active, Archangel's staff will be giving 160AP with end build, Rod of ages only gives 60AP but it compensates on HP and mana. Abyssal Scepter has a good passive and some nice 70AP

With this built you'll have +/- :
2500HP
3500Mana
150Armor
100Resistence
40%magic pen
700AP

TY for posting, glad you like the guide.

As to your question:

Karthus' ultimate is powerful, but he's (in my eyes) considered a DPS mage. The thing is that his cool downs are already shorts...Nearly non existent considering he has 2 damage per second abilities. Defile and Lay Waste. Wall of Pain is a utility spell and the ultimate is only useful when YOU KNOW it's going to get a kill. Otherwise you're wasting the cool down. So building CDR is rather pointless when you consider that Lay Waste is less ~1 second CD and Defile is a toggle.

We're better off improving your Survivability in combination with your DPS, not your 1 teamwide nuke. Does that make sense?

Thanks again for your comment sorry for taking so long to reply, been busy lately.

Nice guide :)

I've been struggling a little (new player and its nice to see a guide with some tactical/synergy information instead of banging on about why you need to do skills in a particular order.

There are a few bits where the wall of text could be broken down a bit though.

Could you tell why you avoid CD reduction on Karthus? Just with the power of the Ulitmate I would have thought it would be reasonably high up on priority. (I did do better with your build than my own, where I focused on getting 35% CDR.)

+1

NJ ^_^

Glad my guide works for you.

Very good build +1 . In my first time with karthus i reach a quadrakill and finish 25/4/11 very usefull guide !
FIRST SCORE WITH KARTUS:

Quadrakill :3

TY ^_^

Nice guide :)

Thank you very much. I'll try, WoTA is disgusting on him ^_^ It makes a lot of sense wit the amount of AoE/Short CD harass he has. Ty very much and keep on trucking.

Very strong karthus guide. You put WoTA into very great use, and I never knew it would be such a strong item. +1, and keep up the good work!

I fail to see what other items I could have built to make him have higher damage with this much survivability. Karthus does amazing damage between Lay Waste and Defile. This build maximizes the amount of time you'll have to damage an enemy team. Karthus is a DPS mage, not an Annie nuke you in 2 seconds mage.

People seem to miss the way a champion's abilities are meant work. Karthus has ~1 second CD on Lay Waste and essentially no CD on Defile (which deals PER SECOND DAMAGE). A wall that slows and a global ulti which has the highest direct damage of his skill set....To build him effectively you give yourself a balanced amount of damage and survival. This maximizes the amount of time you're alive dealing damage...The longer you're alive the more damage potential you have. DPS mages like Karthus/Ryze/Cassiopeia all work this way. Building them for pure burst ensures you won't accomplish your job of DPSing...You'll run in get focused and then your team will probably lose the fight.

Being more durable is more important for DPS mages than doing slightly higher amounts of damage while being a lot squishier.

A champion like Annie on the other hand gets 1 MAYBE 2 defensive items. RoA/Banshee's veil. Why is their more emphasis on damage for Annie? Because Annie is meant to do the majority of her damage in a few seconds then pull out and attack from the outside. SHE is a nuke mage, not a DPS mage.

TL;DR if you're not building balanced survival+damage on Karthus you're doing it wrong.

Appreciate your comment and your vote though ty ^_^

Well, this is quite "support-way" how to play Karthus. He doesn't reach as high AP as standard Karthus but he can obviously support his team by the spellvamp aura and slow. To build this Karthus standing-alone is probably bad because you need some propher teammates to "support" but as "A Guide For Arranged Teams" it's good.