Karthus, GET!

Now all that is left is to figure out how to play the guy. I tend to see him as something of a carry mage, dealing decent sustained DPS, but rather lacking in burst. His AoE is great for farming, but I think I tend to be too agressive with it's usage in a team fight so far.
My current item build is Tears->Rylai and boots->Abyssal, but I feel that I'm severely lacking survivability at the moment. What kind of items should I get? CD reduction? How does straight AP compared to magic pen for him? How do I survive without a tank around to save my behind?

Thanks in advance. I'll be back to check on this in a few hours.

I'm working on a new guide for Karthus at the moment, but here's my current semi-carry DPS build:

Tear of the Goddess
Boots 1
Catalyst
Rod of Ages
Sorceror Shoes
Arcangel Staff
Situational item: Abyssal Scepter/Deathfire Grasp/Void Staff
Zhonya's

I used to go Clarity/Flash, but I've now runed enough mana regen that I can go Flash/Teleport or Flash/Exhaust. I also rune for 10 spell penetration.

Masteries are 9 points in Offense for the 15% spell pen, rest in utility, grab movement speed, mana regen, xp boost, etc.

This build doesn't offer much in survivability, but the idea is you won't be getting targetted since you can harass from a huge range with Lay Waste, and Defile is popped once everyone has already committed in a team fight.

Aha, sounds good. I'll give that a try. (Finally, a new Karthus guide).

I was getting so hopeless that I was going to get Triforce and spam Lay Waste while attacking. From a few older threads that I dug up, it seems that I was relying to much on Defile. I'll be sure to stand far far away and spam Lay Waste next time instead.