Not Updated For Current Season
*UPDATED AS UP NOVEMBER 2010
*5v5 Changed Summoner Spells - Ghost & Ignite instead of Clarity & Teleport
**Also changed some of the gear - First build your Rod of Ages and Boots. Then the soul stealer. If your the killstealing carry keep the soulstealer. If you keep dying and loosing your stacks sell it and continue building Archangels staff.
So, First you need part of the RoA and Boots - Finish RoA and start the soulstealer - Finish soulstealer and start Archangels staff - Build Zhonya's Ring - and once your starting to team fight more and your AP is around 450 sell your soulstealer if your dying a lot. But if you have more than 10 stacks keep it. If at anytime drop below 6 or 7 sell it and try and build another Archangel's staff and keep builging thoes.
Idealy with a good team build you shouldn't reach more than 700AP but in thoes really long games Karthus can be used as an fight starter by using his Ulti dealing around 1k dmg before resistance. If you really want to get ballz sell you boots and get Zhonya's Ring and 5 Archangel's this will give you 1313AP and you will deal around 515-700 with your "Q" ability - Hella dps with your "E" and around if not over 1500 dmg with "R."
***3v3 Basically the same thing as above but will less AP and more focused on Magic Pen and Spell Vamp. The Spell Vamp is still being tested and experimental because I normally dont play Karthus in 3v3 but trying it out. Hit me up with any suggestiongs.
-The first Karthus is the 5v5- Second is the 3v3 item build-
I've been experimenting with Karthus for about 3 weeks now and this is the best set up I've come up with for 3v3.
Your other 2 team mates need to be a combination of these examples.
(1) Strong melee (ie: Jax, Warwick, Sion,Cho Gath, or Mordekaiser) Any champion that can rush/jump in and do burst dmg.
(2) Another caster or a hybrid (ie: Kennen, Kasiden, Ryze, Vladmir...etc)
Champs that can stun or to more burst dmg to allow Karthus to activate his 3rd ability (His mana draining AoE)
I personally like the Karthus, WW or Jax, and Kennen. This allows Karthus to be the initiator and finisher while WW/Jax and Kennen do the work.
For gank set up:
Have Karthus initate the fight, when the enemy gets greedy and commits set up your wall. Hopefully they with take the bait walk through it or place it behind them so when they run away WW/Jax & Kennen can rush in and do clean up. If some one is getting away, nuke them with your Ultimate, ability 4. This should only be done after lvl 6 to ensure all your team mates have their ultimate.
As for items your build should start with:
(1) Mana crystal and some health pots.
(2) Boots and more health pots
(3) Either finish your boots -or- further the staff with the 7mp5 rod and whatever else you can buy
(4) Depending on who your fighting get the [chain vest] for the sunfire cape -or- the 48 magic resits item
(5) Finish your armor item
(6) Start stacking AP [ability power] items
(7) Engage the win button =D and enjoy, your now a force to be reckoned with and if you do die you should be able to bring them down with you
***Amendment to this build [October 7th, 2010]
This build focuses on stacking AP to: Nuke the enemy team in a big skirmish, and to finish off and stragglers that are trying to runaway and heal.
To start you want a mana gem (blue crystal) and build up to an [Archangels staff]
Next get some boots, I prefer the boots that have magic resist.
After this if it looks like it is going to be a long fight use the second item build and stack Archangels Staffs. I know the 3% AP bonus will stack (3%, 6%, 9%, 12%) In order to obtain 4 staffs you need to have a ton of gold when you purchase the last one and may have to sell an item to get these but your gonna want to.
[Things to keep in mind]
One think I like to do just to get an extra boost is leave an open spot in my inventory and use Elixirs of Brilliance. Great AP boost early in the game.