Thanks for the suggestions, I really appreciate it!

I know most people don't like to buy Athene's Unholy Grail, but I consider it a really cost effective item, even without using the mana regen. I did mention that other items such as Will of the Ancients will also benefit Karthus greatly. That's mostly just my personal opinion though.

As for Requiem, I think it was my wording that was the problem. I meant to say something like "A common misconception with the usage of Requiem is that 'You have to get a kill', but that is not always true." I think I'll change that up a bit.

Again, thanks for the suggestions :)

Good guide, well put together, etc. I would like to point out a few things, however.

First of all, Athene's Unholy Grail is not good on Karthus- He has passive mana regen from Defile, and his Lay Waste has very cheap costs. I use it on on champs who have bad mana regen and rely heavily on their mana, like Soraka and Anivia.

Also, casting Requiem ro secure a kill early on isn't a bad idea. Late game, this is not the best idea, as you WILL need it to win fights, as you mentioned, but early on the more or less free kills (and the assists your allies get) can set your team far ahead.

Overall, excellent guide, I upvoted.

With regards to the Brute Force and Butcher . While auto attacking to farm isn't optimal, it doesn't mean it can't be done. Even with a hella slow attack animation, you can still auto attack for a bit of early harass.
As for the item order for Rod of Ages, getting catalyst the protector first will grant you better survivability in lane, as opposed to damage. Honestly, I'm more of a "safe" player, so I find that the health benefits me better than the early damage. This is dependent on the players themselves though, and I think I should note that in the items section.
With Athene's Unholy Grail, it's true that we won't need the mana regen with our massive supply. However, it is one of the most cost-efficient items in the game, IMO. If you don't want to get that item, I have listed some alternative choices in another section.

Just because there are some things, that aren't optimal doesn't make it a bad guide.
Have you even read the text below or just looked at the cheatsheet and said "wow I don't agree with that, let's go downvote", It can be updated you know? It doesn't have to stay in this suboptimal state.

So the guide is wrong, but you pity upvote because of the effort?

I really like the effort you seem to have put into this guide.
Although I do disagree with some of your itemchoices, like Athene's being so late (why not got for sth else, since you have already stacked alot of mana through RoA and Archangel's)

I don't like your masteries and item order.

If you want to go the roa route, that's fine, but I take issue building a wand and shoes before catalyst. Also I don't like brute force and butcher on Karthus. I find it pretty easy to last hit with aa's when I have to. Also Athens as a last item is a very poor choice.

I do appreciate your reasoning on a lot of your choices, I just find it incorrect in a few aspects.