Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is my first build, but please try it out before you judge it as I use a differant approach to karthus. I also want to put this here don't be a jerk and just - my build without comments or trying please.
First a disclaimer. As I am a medium player this could be a medium build, for those of you that either have a similar play style as me or who aren't yet pro this build will work great. My build late game revolves around getting right up in those dpsers faces and smashing them hard. I choose those style becuase of a few reasons. First it works, if I find people like this build enough to vote on it and such I will put actual stats up here. HOWEVER PLEASE NOTE you may not be a huge upside k/d if you are one of those people scared of dieing DO NOT PLAY THIS BUILD. This build is a HUGE support build, usually in my team fights me and maybe one other person have died while gaining an ace. This is huge since now they have upwards of a minute to push. I also choose this build becuase you are no longer squishy targetable karthus, now everyone will run from you in hopes of not finding out what the 330 per second defile mixed with 350 per second bombs is. Late game there are now no heros (unless they are ungodly fed) that you should be afraid of.
Now that that is out of the game I will do a basic overview of how I play.
This is hopefully a very fast time period as you should be middle, there aren't many heros more dependant on middle than you. Harrass slowly with bombs at the start to find out how your opponant moves so that later in the game you know how to place your mines. once you have your 2nd lvl bombs start taking out the caster creeps should take 3-4 bombs. Unless you feel the need to stay go ahead and port back buy your crystal and boots than tele back in. Kill off whatever creep wave you have there to start the push back than go kill the golem (assuming you don't have a bunch of junglers, if so just stay mid)
If like me you are not good at ganking this is going to be your hardest time. If you can gank please by all means this is when you should be doing it, all you need to do is hit your wall and planting mines at peoples feet is as easy as sleeping. If you are like me and have issues here try and power lvl through this period, wiping out whole creep waves with defile and mines, while watching for times to either pick off opponents with your ult, or helping your team-mates get a kill with it. PLEASE DON'T K/S it is a waste of a great ult, and in the long run who cares if you have 14 or 20 kills becuase you stole them, let your team-mates build up. During this time you should save up the money to get Rylai and your rod. Once your have Rylai you can now call yourself god.
Welcome to godmode. Now all you are gunna do in fights is attract attention and kill people. At this point you will be doing about 200-300 dps with defile, mana shouldnt be an issue (especially if you keep killing golems) if you dont have max items yet buy the brilliance elixirs to give you an extra punch. Make sure your team-mates know to fight in your aura and you will kill all their team before they know what to do and you should have atleast 3 of your team up still to push. Your bombs and aura plus ult are all great ways to catch runners if need be since they all slow with ry's. I would say get in the middle of thier whole team and than use bombs on the outside casters/range dps, or if you are real high on the ap side, blast through the tanks and melee dps. Once they all target you (which will be quick) use zonya to avoid death for a few seconds, than when the inevitable comes just fight through it as if you were still alive. 8 seconds is a TON of damage. Even if you died as soon as the fight starts you can average 1600 damage with your aura in 8 sec if not more, and 1700 with bombs if not more. that together kills anything AFTER you die.
I hope you enjoy my new approach to Karthus. Please don't rate me bad or dis my build becuase it is not a 2.0 k/d ratio build. This is for medium - pro players with karthus or if you are just tired of the squishy hide and try to k/s builds. Any help with this build along these lines though I will greatly appreciate as I am only about 5 weeks into this game.