Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Karthus is the bane of most champions. This build makes you a complete ******* to the other team. You will not necessarily get a huge amount of kills with this build, but you are not likely to die more than 3-5 times, and you will be a HUGE asset in team fights.
Why teleport and ignite? Teleport so you can go from b, to lane quick. Ignite, to kill enemies early game. And to take half of the healy champs heals away, cuz thats the kinda guy Karthus is. Other good spells for Karthus would be Clarity, Flash, and Ghost. None of the others really work that effectively.
So we start out with our mana crystal and as many health pots as we can and head mid. Your mana crystal should keep you well manaed early game. Once you get your Catalyst the Protector (1st trip back), you are pretty much unkillable (if you play well), until mid game. Then we grab our sorc boots (hopefully on the 2nd trip back). At this point you are looking pretty deadly. This build is not about early game kills. You would be fortunate to grab 3 kills early game. Once you hit mid game, you will be a force to be reckoned with. So now we get our Rylais. Buy health first if you can unless you can buy both AP items in one trip. Once you get Rylais you are ready to start pwning. Time for Banshees! Karthus is easily CC'd, sou getting Banshee's is a must. It gives you enough survivability to take being targeted for a bit. Get Archangels for the nice mana boost and AP. Then Manume. Once you get them both, you are gaining max mana like crazy! Now that you have all of your items, the game should be over. I think this is really the definitive build for Karthus. It gives him the AP he needs and the damage to take down turrets.
Grab your mana crystal and a health pot and head mid. You should try to last hit minions as much as possible with your q ability. You won't run out of mana no matter how much you use it, especially once you get your AoE abilities passive. If you are low on health or mana b. Don't worry about what you can buy, just b and teleport back when you need to. Farm like a mad man. This build requires a longer early game because you NEED Rylais before you can really do much ganking. Use your ult to help other lanes and if you failed at farming or died too much early on, don't be afraid to ks. It will benefit you most in the end.
Once you got Rylais you can actually start killing! Your main damage dealer is your AoE Defile. Use Lay Waste for extra damage, and if they are getting away, wall them in for a sure kill. Be ganking and farming (never aim to kill towers on your own). You are useless for actual legit tower pushing until you have manume.
You have all your items and you pwn serious ***. Use you awesome to kill and push. The best of luck to you!