This guide is all right, but there are WAY too many big chucks of paragraph... -1

good build

Good guide. Nothing ******ed.

If you want good survivability Karthus, consider Rod of Ages.

I like your build. Personally I like to use the spell Surge and then use Karthus Ult. I understand you did heal for survivability which I tend to use sometimes anyway because I suck at PvP. The first item you go for is tear of the goddess and at lvl 2 having defile giving you that extra mana per kill, I'm not sure if you really need to have the mana seals. Great guide! :)

Thanks for response - Actually I tested it.
Upgrading into Archangel's keeps all the bonus mana from Tear, and as You keep casting spells and increasing your mana, your AP also keeps increasing. Therefor I see no reason not to buy Archangel's early.

I am not really a Karthus player, but if I were to build a Tear of the Goddess, I would not upgrade it into an Archangel's Staff until end game or until I maxed or close to maxed the mana bonus it gives(because your mana you earned will be tranferred as you upgrade it).