Bronze noob need karthus nashors advice.
So I was playing Karthus a lot recently because I really like his ult and his Q and I really like getting multikills with the ult :D
So my build is as follows (not in any particular order):

Here's the problem.
gives good AP and 20% CDR to complete my 40% CDR (which I need to use my ult because I love it). The problem is it comes with an attack speed increase which I never use and a passive for my basic attacks, which I also never use.
The problem is, if I want to keep
for full magic pen, I only have one item to complete my 40% CDR because I can't sacrifice
, since it is too much AP.
So is it worth having
just for the 20% CDR? There's no other item with 20% CDR that has good stats or usable passives besides
which I already have.
Obviously it's a somewhat gimmicky build made for high ult damage, but I feel like in my low league I can afford to have some fun with it. However I'm not sure about the
, maybe it's making me severely weaker than if I said screw it to the CDR and got
instead of it and maybe even
instead of
. Any help is appreciated.
I also have 15 MPen from runes, so maybe the boots are overkill?

and
aren't that bad of options when you are looking for CDR. It'll bring you up to 30%, and you can get the last 10% from your Blues/Quints. Protobelt is especially fun to play with, since the dash allows you to get into position better, while giving you some decent AOE damages. Quite a bit of people, at least from what I've seen, will tell you to build Rylai's. There's nothing wrong with that, but its a pretty terrible item, stats-wise, and when you're skilled, or in an unskilled environment, you don't need it.