I think this is outdated.

This is ridiculous... I've been on 3 different websites on builds for Karthas but you guys overlook certain things. Heres my 2 builds: one for damage other for damage and health sustain. Seraph's Embrace, Liandry's Torment, Rylais Crystal Scepter, Void Staff, Rabadan's Deathcap, Zhonya's Hourglass.        ...OR...                               Seraph's Embrace, Void Staff, Rabadan's Deathcap, Zhonya's Hourglass, Deathfire Grasp, Will of the Ancients. ( High spell damage, high healing per spell... Especially for R and spamming Q so health is made up for)

 This turns Requiem into a nice sized heal  

Guides are just that. Guides. They are not the end all and it would be impossible to give you a build for every situation and teamcomp you could possibly ever want to use a champion in or every teamcomp you would ever want to devise.

Could you use Seraph's on Karthus? Absolutely. But it's a build that is riskier than a RoA build and has a longer build up time. You also suggest a build that is quite possibly one of the most expensive builds and takes an extremely long time before you get any level of power (Liandry + Rylai + Seraph). So you're making very little impact early to mid game because you're just sitting there trying to farm and charge up your tear. There's a reason it's not in the guide because your suggestion is stupid and doesn't understand the purpose of Karthus. Sure, if you want to afk farm and somehow convinced the opposing team to not do anything also for the first 30 minutes, knock yourself out.

BTW i wouldn't suggest going RoA AND Seraphs embrace.  Karthus isn't Ryze...he doesn't need 3400 mana.   You do get +96 ability power from stacking RoA with Seraphs (154 AP for 1 item) and an 850 hp (max) shield, but the permanent 35% slow and 138 AP (60+78) is more useful.  I guess if you're intent on *diving*, then RoA, Seraphs+Dcap/Void will do a lot of damage.   But again--the longer you survive in fights WITHOUT dying, the more damage you will do.

I honeslty think that, ever since the spell pen changes made %pen come before flat pen (very important change) and the new items, that RoA is simply not as useful overall nor as cost effective for the utility it gives.  Haunting Guise is very strong on almost all AP carries now, and helps Karthus hit like a truck, even more so if you use spellpen blue runes as well as reds, and instead of RoA, you should rush tear, then Rylais, and then depending on team comp, either complete the AA staff into Seraph's embrace (650'ish HP shield if you have full mana) or rush Zhonyas (only if you REALLY need the armor, as completing the AA staff is 2000 vs Zhonyas from scratch is 3300 (?).   You'll usually want to complete the zhonyas first if you started armbands e.g. Karthus Top or mid vs Bruisers, or if you're in danger of being instagibbed and the serpahs not being strong enough.

I know Scarra feels Rylais is somewhat questionable, but there are some very important things to consider here.

1)  You can't always rely on your wall being available as a CC as it has a 17 sec CD without CDR.

2) enemies and gap closers can jump over it, then you're forced to engage.

Instead of RoA Dcap Void Zhonyas, a Diamond 1 player (Russl) prefers a core build usually consisting of this:

Rylais, Seraphs Embrace, Zhonyas (unless going tele revive zombie karthus, since Zhonyas is less useful if you don't have flash, then deathcap/hextech is an option), Liandry's Torment (a natural Rylais pair), Void staff, Sorc/Homeguards.  This is even more attractive since Zhonyas now gives 120 AP and Serpahs Embrace gives around 136 AP AND a shield, thus giving you TWO shields.  Deathcap gives you 120 AP+30% but 30% doesn't scale well early game, so putting all that money into an early deathcap when you can have a far more useful Zhonyas Seraphs makes a lot more sense.

This build basically allows Karthus to simply destroy what are usually some brutal matchups, and allows him to control where engagements take place and a permanent, spammable 35% slow can kite an entire team.  Champions like Singed, Voli and many other tanks/bruisers basically get countered, and many matchups become much easier for both you and your team to handle.

Also, a void staff will hit -harder- than a deathcap if they have 80 MR (spell penetration and the AP), and if you use spellpen reds and blues, you will do true damage to anyone with 80 MR or lower.  Do not underestimate how important spell pen is!  Void+flat spellpen (besides sorcs) didn't really work well in Season 2, but now with % coming BEFORE flat, its a strong combination.

Remember, the longer you survive in fights or pokes, the more damage you do, so having 600 AP and the ability to protect your AD carry and yourself a lot easier, is more important than having 750  AP and having to rely only on your wall.

Just remember not to rush Torment.  Rush Guise then get your other high damage items and grab the Torment when they actually start stacking HP.

Try it....I've gotten some completely absurd scores with the Rylais/Seraphs combo, and even a nice recent Penta :)

RoA is still useful in some matchups where you feel you REALLY need the sustain and the hp all at once, and yes, you do get damage sooner, but you can always grab a chalice/grail if you really need the MR (maybe substituting guise, etc).  I'm not saying it's a bad item...it isn't "bad"...but isnt having a PERMANENT 35% slow Q kiting skittles of death far more useful?

Roa is far superior then any item to start with.It actually the best.Sure you may get more utility from rylais but you will die to ganks more easily,you will be severly weak on your mana pool, and plus rylais doesnt help your farming and sustain.If you can get your rod before 13 minutes it better than most items that you can start with.Farming faster and longer and not dying>35 percent slow. 

I'm fairly certain that every pro player starts Tear on Karthus now (Tear -> Catalyst -> RoA) and then expands from there depending on playstyle.

The Koreans go Zhonya's or Abyssal after Tear/RoA since they are a much more aggro with TP Karthus (think RapidStar) while the NA/EUs tend to go more global passive/press R Rabadon's into Zhonya's (think Scarra/NyJacky/Froggen).

Is it a good idea building hem more HP, with like Rylai's Crystal Scepter for example. With Liandry's Torment and with his E will deal a lot of damage. Is it possible build or it has some issues that I missed?  

thx for fixing, i realy love this guide, ive had a hard time picking who to main in ranked soloQ but after this guide i realy fell in love with the lichking of LoL.

Before i used to have way to many champs focusing way to hard on counterpicking, now i just go Karthus vs most and instead of counterpicking i counter with runes masteries and items, this way you get way better faster focusing on less champions.

THX! for the inspiration

In the quick guide it says Magic pen marks. but in the masteries and rune section you say magic pen marks is wrong ? so what is it ?

I get rod of ages for a reason.....because with it comes a lot of extra mana which is good to push lanes when they are full of creeps.....when the towers are down and the creeps are almost to your base Karthus is the BEST at clearing those minions fast...he can go full speed thru a pack of minions and kill them all without stopping late game...but not if he runs out of mana. The other reason is late game I like to tower dive and hit Zhonyas hourglass and force the enemy to run and give up thier tower or die...because when I die I ulti and get triple or quadra kills...It may seem reckless but sometimes my sacrifice means winning the game because a late game triple usually means the game is over.....