Build Guide by De\/\/o

Foreword:
Since the last patch, Karthus can be demolishing when build correctly. Ofcourse this build takes long to finish, but when you have finished it once you don't want to use another build ever!

Items: It work's just like Veigar, Archangels FTW. First you get the Doran's Ring which now adds +15 AP and a HP pot. Second you get yourself Sorcerer's Shoes, this is the best to choose for Karthus. Then start building your Mejai's Soulstealer, If you're not doing well you could also go for Rylai's Scepter, but I simply advice the Mejai's. Then get your first Archangel's staff, Then Zhonya's, Lich and Archangel's. That should speak for itself.

Masteries: I go for the regular 9/0/21. With upgraded Teleport and Clarity.

Summoner Spells: Ofcourse I go for Teleport and Clarity, so you can harras your enemies longer and go back to base, without any problems.

The runes are standard: Magic penetration Marks, Flat Health Seals, Flat AP Glyphs and Magic Penetration Quints.

At early game you might be a little bit squishy, so try to play more defensive. But at late game you will soon hit 1000+ with your ultimate and over 800 with Lay Waste. And this will be even more if you have Mejai's, so make sure you follow your team in team fights and cast Defile to get a hit on every champ, even though they're just assists it gives +1 which is fine. Or you could just Ult them when they're all low. The Mejai's is very good because of Karthus' passive, as soon as you die you can cast your ult or keep casting Lay Waste on a single enemy, which will deal alot of damage and will count to your stacks.

The build actually speaks for itself. I hope it will help you as much as it helped me!