Intresting but isnt 15% cd reduction better than 15% magic penetration or 10 ability power :S

i wont vote :)

Karthus is Karthus, not Morgana.

She doesnt NEED the Rylai since she only has a nook if you place a Binding on the enemy, and if you do... he is immobilised anyway so slowing an immobilised enemy?

I can agree tho that it helps with placing your ultimate. but i would still prefer using Rod of Ages for the HP. Its cheaper.

If we're talking about casters in general, then I must say I find one item quite helpfull with Karthus. It's Rylai's Crystal Scepter. It has two things besides the gnormous amount of AP that help. First one is +500Hp. It just helps you survive longer and has saved me lots of times now. The second one is the slow down effect. I know, Karthis has ***, but it has quite long CD and quite often you don't have it ready yet when someone's fleeing. Now the slow down effect of the scepter let's you bomb your enemy into oblivion fast. And since Karthus' bombs have a delay, slowing down your enemies help with accurate placing of those bombs.

As for zhonya's ring - it's darn expensive and I rarely get enough time to buy it. Not saying it's a bad item or not neede in this build, just that sometimes you can buy something else if you're not sure you'll be fast enough to get it.

Hope this wasn't a paain to read due to mistakes and whatnot (English is not my native lang).

As you are right Rooke on soil thing... damn its GREAT spell for farming and even harrasing early game, pushing your enemies back while getting free creep kills :) I can understand getting ulti last last, i find it extremaly situational spell :/

In items, i feel like you overdone with MPen, boots, void, abysal? I would get something with even more AP and skipping either abysal (if you have tanks with MR) or void if you face no tank team.

looks good for the most part. At one point you said:
"Because your income isn't really based so much on killing creeps"
I totally disagree with that. Tormented soil is arguably one of the BEST farming tools out there. Also I would get black shield early on, sometimes at level 3 depending on who I am versing because it is an amazing defensive spell and my brother has saved my life numerous times with it. In fact, one time i could not see anything on my screen because the whole middle of my screen was lit up with the spell block message. Also, you probably want a third point in your ulti at 16. Your masteries address her cooldown issues which is good. All in all, not bad.