i would finish the build -> glacial shroud-zhonyas ring-frozen heart

Swap frozen heart for rylais unless very autoattack based opponent, and abyssal scptr if magebased. Then its quite much the same build I use. ^^

Dude ur build PWNS!

I don't think this is good build.. tried it and its fail... Karthus cant do anything with this, 448 is low ability power, with some better builds you can deal 1500 damage per second so you dont need armor and mana from Frozen Heart, the only thing why this item is good is cd reduction and boots of swiftness on Karthus?!? you really dont need them.
The only 3 items what I agree with are: Archangel's staff, Mejai's soulstealer and Zhonya's ring
About runes: WTH?!? armor and magic resist?!? I know his armor is low and he has low hp but you really need it when you have 1000 ability power.

From my side its very bad build.

I like the build, though I disagree with the frozen heart before zhonya's. I prefer the nearly 200 AP and the invuln. If you are wanting the CD reduc, which personally I'm not a huge fan of on kathas, get nashor's or something, at least it has AP.

Main point I wanted to make in this post though is this: ALL good Karthas player should know this, and do this. At the beginning of a team fight, right when things are about to start, or have started, hit your ult. I know this sounds dumb, and morons will say, "why did you hit your ult so early noooob! Now two are barely alive running!" But what they aren't realizing is why your whole team is still alive! You started the team fight with a 3k+ hp buffer! How much easier is it going to take down their whole team if they all have 800-1000 less hp?

Seriously though, if you are not a noob, and not just totally selfish, you do this, and your team will win, not just you.

Frozen heart over rylai's anyday. it gives you mana which will give you more ap due to your archangels staff, a **** ton of armor, and the cool down reduction is far better than the slow because you should be able to kill someone being slowed by your wall before the slow goes away if you know what your doing.

I like this build a lot, it's very similar to the one I run commonly. There is just a couple things I would change, one being I almost always get my zhonya's before the frozen heart. Both the frozen heart and zhonya's are really expensive, but by the time you get to building one or the other, you are sitting at 300-350 AP and just from getting zhonya's you start pushing closer to 600. That is an incredible difference. On top of everything, you probably won't get both by the end of the game, but you can get zhonya's and a good chance at the glacial shroud at least.

If you are needing CD reduc, just go kill the golem.

I also don't really agree with rylai's the slow is all but useless, and there are better things for survivabilty and AP out there.

Really good build. Played 3 times with Karthus.
First one was 5/2, second 2/2 (my team got only 4 kills), and third one was 21/4/13.

I guess when it comes down to it though its frozen heart or rylai's and the frozen heart seems to benefit more just a matter of taste i guess?

I would actually have to agree on the Rylai's yes the slow might not be a HUGE benefit but please take into account the huge burst of HP it gives and AP. IMO Survivability is just about equally as important as dishing out damage considering the range of his most damaging spells. You usually have to get up close and personal with heroes if you want to dish out huge amounts of dmg.. so yea i would highly agree with sticking rylai's somewhere in that build. Good build though Thumbs up from me

I don't think so, You can only get a 15% slow with Rylai's Crystal Scepter since ALL your Spells are AoE. Not really worth it imo but it could be fun ^^,

But if Lay waste triggers a 35% slow on single targets then im all Rylai's, if not then it's a good optional.

btw, I don't think slows stack... say: Wall of pain + Rylai's Slow = Wall of pain

It will take the biggest slow first, then when the biggest slow runs out it will take the longest, which in both cases are ***.

You should replace something with Rylai's. Rylai's + Defile = win.