Hey!

This is my first build/guide, so please be nice!

I have played Karthus as my main for quite some while now, and this is the build that I use when I play him. At a normal game, this build shall only take 45-50 minutes to finish, and therefore, you do get alot of cash after the build is finished. I use this gold to buy elixirs to mainly gain even higher AP, and some more hp etc. Even though you might have 6 items, you can still buy elixirs (consumed automatically when bought).

There is some things you GOT to know/should respect when you're playing as Karthus:
1. Do not use the ulti to steal kills, only if they managed to get away in the other lanes.

2. Tell your teammates when you got ulti, they can get you kills, feeding you early game, making you a killing machine in early/mid game.

3. Karthus is a "suicide bomber" in teamfights. He enter the fight right after the tank, activating his E (Defile), spamming Q (Lay waste) and using his W (Wall of pain). When you die, you use to passive to activate your E, and to use the ultimate. Remember to use it early in the passive, so you get most kills/assists, and so that you do not miss your chance (it is important that you use it the first 4 seconds of the passive).


More is to come, this is still a fresh guide!

Laning:
Never go agressivly before lv 3. Poke enemy champion before engaging. After lv 4, slow with W, activate E and spam on with Q. If he gets away with low hp, u use ultimate.

Who is impossible mid:
-Leblanc
-Kassadin (mainly after lv 6)

Who is hard mid:
-Akali (rarely goes mid, but it happens)
-Annie
-Brand (if good at skillshots)
-Cassiopeia (if good)
-Ezrael
-Katarina
-Kennen
-Lux (easily countered if taken out when abilities on cd)
-Morgana
-Orianna
-Also many ad carries.