Rate my Karthus Build

Hello everyone here is my build and a reason as to why i build it. I tried building mostly damage before but people would just dodge my Q is I didn't use my wall. With this build I do more in team fights and still affect tanks as well. Please say which you would change and, why, what you would build instead, or just say nice build and the try it out and comeback and give your thoughts on what works and doesn't work.

: Really Strong Item on . Turn On E and permaslow, Q someone and land another to slow again, need to slow an enemy to help your ally, ult and slow. Gives health too.

: Combined with the slow you get the 4% and do lots of damage for team fights even when you die it still applies. Gives health too for survivalist .

or : I've built in the past and the passive abilities of the other two seem better IMO. Either I need more magic resist and tenacity against a CC heavy team or the cool down for my ult and spam Q's faster to proc the slows given by another item. I get the red upgrade because it lets me run away or towards an enemy with my E on so a bit of engage and disengage.

: Standard Karthus Item for mana and gives a nice shield too.

: Standard Karthus build. Gives nice amount of stats for early to mid game and fully stacked it is still good too.

: I don't know what to build last so I usually go with this. I don'y want to build because I feel the enemy team would just build MR against me and the 35% MR pen works nicely with .

So that is the build I am doing now with so far a high success rate. This may not be the place to just say "Here is a build try it" but I mainly want to know if there are any huge flaws or downsides to a build like this.