Hello there, I'm Karthus main since his rework I think.
This is a build I use almost every single game! (RoA,A Staff, Rabadon's...)
I think this build is one of the best for mid lane (mid elo= gold, plat).

Early you will try to save your mana for engages and that kinda stuff. But you will still farm with your Q (JUST TO LAST HIT). If you don't last hit, it is useless. Level 2 you need to last hit and your E passive will give you 20 mana back. Q is so much stronger if you hit only one thing (creep, champ). Late when you have Seraph's already you can just Q almost infinite. Your E is used early and late to pick up low hp creeps that you cant AA or you just don't want to use Q.

KARTHUS IS A GOD CHAMP!
But you gotta learn him, like every single champion in the game.
Passive - No CD, You can go there and die in a teamfight and still get a penta (7sec duration)
Q - OP ability if used right.
W - AoE slow that can slow enemies from attacking you and if you want to easier hit Q you can W them to slow them so you can land Q easier. (W is also used to check bushes because the ends are like the wards)
E - AoE damage that uses a lot of mana early, but it is OK because early you used just in fights/engages/sometimes lats hit creeeps
R - OP abilty but you have to time it and if you get cc-ed or moved ULT WILL CANCEL! And it has 200 seconds CD and that is so irritating!