"How do you play Karthus?" - Karthus player
"You just press ARRRRRR" - Gangplank player

This is my 2nd build, but my 1st "hope to be" good build. This is the way I play karthus but you cant just follow the build and expect to do good. You must practice with the champion, spamming R just wont work with Karthus after the Jayce patch, well if you dont got CDR

Pros:
Very Powerful
Good Harass
Very good at Chasing with out Chasing (ULTIMATE)
Excellent Crowd Control (Slow)
Can kill after death (Passive)

Cons
Squishy Early and Midgame
Needs to play Mid
His Lay Waste takes time to master (Q ability)
His Requim Requires you to get close but you can't because you are squishy

This isn't a joke Karthus is a very good jungler but he relies on blue or you will run out of mana insanely quick, this is a mid guide not a jungle guide but keep in mind you may want to try out jungle karth, just keep turning on and off your E spam your Q and start with cloth armor and HP pots. Start off with a leash at blue, wolves, wraithes, golemns then recall of ask for leash red. Then make sure you have your wall and GANK Karthus has a very powerful Gank becuase of his wall

You need mid, call it in the lobby. If you dont have mid you aren't going to do any good. When your in mid don't use your Q unless your only going to hit one target which would be the other teams mid champion. Don't spam this until you get your defile then you must last hit with your Q to get your mana back. REMEMBER FARM FARM FARM

This is where some ganking happens. Remember by now you have your ult. Always be looking around the map for fleeing and low hp enemy champs to finish off. Your ult is how you get a large portion of your kills. So get somewards put them in your bushes and i don't suggest ganking because you are very vulnerable up close, the last thing you want to do is have a confrontation in the river or jungle, it will not turn out pretty. But do remember you can ask for a gank from your jungler but remember wall of pain is an amazing slow so use it in the gank. Don't forget about your ult too.

You should have most of your items by now or at least have build into the codex for its CDR. Stick with the team as your can't do much damage up close and alone. It will be a useless death but if you stay with your team your team you will be able to do "tons of damage", if you need to do lots of damage have your tank taunt (Shen) or stun (Amumu) and then turn your E on and hop in the middle while spamming your Q. But also remember you have Zhonya's hop in with E and use its active and your shouldn't be very squishy late game with your Rod of Ages and Rylai's.

This is my 2nd build but my 1st good build if you have any suggestion please comment