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.
Ok. Before you criticize my build, first read my explanation below. I chose 21 defense over utilty because of the Teleport Spell and high amount of mana. You will not be needing HP and MP regens in Utilities. And ofcourse because in the heat of battle, your defile needs to hit enemies so you need to be closer and a lot tougher. Karthus is a very squishy champion if not properly dressed.
You should be soloing mid. Buy Sapphire Crystal at start and 2 pots. Try to harass your opponent early with lay waste even if you run out of mana. Your goal for now is to harass him and the same time last hit creeps with lay waste. A good Karth player can lay waste to death at level 3 or even at level 1. Try to anticipate enemy's move and go for the 200% damage of lay waste (done by hitting him solo without any creeps hit). Remember, you are tougher with Tenacity mastery. By the time you deplete your mana, you should be around lvl 4-5 and should have more than 580 gold. Both depending if you got first blood. Recall back to base, buy Tear of Goddess and some pots if needed and Teleport back.
Now since you are solo. Once you reach level 6, be more observant of other lanes. Do not leave your lane and continue harassing and farming. Tell your team your ulti is ready and ping and champion who you think is the easiest to kill. Whether its a kill or assist with requiem is ok. You get gold too in assists. Your max mana will increase dramatically later on with tear of Godess this early in the game. Your next goal is the 1300 gold mark for Meijas. By this time your TP should be available. Just recall, buy and TP back to mid.
Continue getting those assists and kills without ideally leaving your lane and keep those stacks coming. You can get blue buff by activating defile and 200% damage golem. your mana would be depleted but will recover easily.
Next get Rod of Ages. This will give you HP for survivability, mana for longer defile, and AP for more spell damage. This will increase also overtime so this is a GREAT item for Karthus. YOu should be continuing gettng those stacks up by killing or assisting.
Get Zhonya next and see your AP jump. Then Archangel to see it jump even higher! your mana should be 2300+ by now and should be nearing, if not yet having reached, the 1000 maximum mana of your Tear of Godess. By the time you get Archangel, depending on your stacks, you should have 500-700AP now. OUCH!!!
Next and final item now depends. I usually get Rylais for the extra 500 hp, AP, and slow effect. But sometimes I go for Void Staff for Magic Penetration. If your stacks somehow got low, you can sell meijas and buy both. Don't forget the Elixers.
This build will make Karthus tougher with your 21 in defense and extra HP from Rod of Ages, makes Defile longer with also Rod of Ages but more importantly your early Tear of Godess, and spells stronger with a ton of AP from Meijas Zhonyas and Archangel.
TRIED AND TESTED>