This is a great build for dominion. It essentially prolongs Karthus survivability by taking advantage of the fact that many of your targets will be stationary or in confined spaces, and thus easy to hit. The AP ensures you'll do lots of damage to enemy champions, especially because there aren't many minions around and your Q skill therefore does DOUBLE.

What to buy:

So you may be thinking: what's with the redundancy? Well, see, key to this build is maximizing SPELL VAMP to 40%! This means that as you hit your enemies, you'll be healing yourself constantly as your E skill swirls and your Q explodes.

I recommend the Archangels staff for not only it' high AP output, but also it's high Mana output and Mana-regen abilities. As all of the game is spent fighting, you need your mana to keep regenerating like MAD.

Sorcerer boots and Void staff are self explanatory: you need to do more damage to get more benefit from your spell vamp!

Alternatively, if you want a bit more health, you could get Rod of Ages instead of Void Staff or even Archangels.

Flash: to get the heck out of there or to get into a better position to die. Karthus' passive allows him to still kill people for 7 seconds after dying, so he's great for holding a turret.

Promote: really, you can get whatever other spell you want. I like this because it helps me take their tower or holds 'em back off mine.

In short, this build will ensure that your enemies will have a VERY difficult time taking your turret. I recommend a more defensive style, as Karthus' W skill lends itself to that, and as you can't run around AND Q-spam your enemies. Karthus makes a good turret.