Main Build (Aggressive Damage Support)
Hello everyone. I've been playing Karthus main for about almost 3 years. Karthus is hardly played anymore with all of the new champs coming out that seem very skilled and have high damage and crowd control. Usually Karthus is played mid lane due to his high ability power damage and Lay Waste range. So you may think right now, "Karthus support? Is he insane?"......I tell you right now friends, I will prove you wrong. There are 3 different kinds of supports, utility supports, defensive supports, and damage supports......Karthus is damage support all the way. While he lacks in sustainability, and only had his wall for any kind of crowd control, he has the best poke in the game. The range of his lay waste will keep his bottom lane away from farming, and help your adc push and get as much creeps as possible. First priority is to ward the bushes so you can see where the enemy is at all times. Now that you have vidion you can just spam your Q (Lay Waste) and keep them away. With this build you will do tons of damage and prevent their adc from getting any farm. With Karthus Support I suggest taking Heal and Teleport as your spells. Most people would say "What about Flash?" Well due to Karthus's passive of staying alive for 7 seconds after a death, he is more of an aggressive player that wont need to flash away, and instead engage a battle. You also have your wall which will slow them to help you and your adc get away. Early game you want to poke as much as possible and help your adc get farm.....but after the first turret is destroyed and everyone begins to roam the map, you can build Karthus as you normally would if you were mid. I've played Karthus support many times and have to say that I have actually carried many games. Because you build into your AP build mid game you basically have a support that can do a ton of damage, and because of your passive, you are a huge help in team fights. Give it a shot and I promise you wont be disappointed =)
Pros / Cons
:Best Poke in the game
:Your passive lets you destroy in team battles
:Your ultimate can help you get kills no matter where you are
:Your wall will prevent enemies from escaping or save an ally from getting charged by an enemy
:Surprisingly tanky late game with build above
:Defile will help you farm very easily late game
:Defile also makes Karthus the best late game lane pusher in the game
:Minimal crowd control
:Squishy early game
:Players will get mad at you for carrying games as Karthus support =)
Creeping / Jungling
Karthus is also a great ganker. Most people that see Karthus coming will immediately get the **** out of there! Gank a lane that's pushed, throw up your wall, activate your defile to do damage per second, and keep spamming your lay waste. Guarantee you will get a kill!
Obviously as a support you do not want to take any creeps away from your adc, but mid-late game once you and your ac have parted ways, take advantage of Karthus's Defile and split push wherever you can for maximum gold and buying your full build as quickly as possible.
Karthus is one of, if not the best team player in the game. If you get stuck in a 5 vs 5 battle, Karthus can go right in and tank the entire fight because your defile and wall will cause their magic resist to drop as well as their speed, and do massive damage per second. Plus if you die you will have 7 seconds to continue casting and help your team. Personally I usually wait until my ult is up to dive in and engage a team battle just to ensure that you can clean up the fight afterwards.
Remember to keep your defile off when not in battle because it will drain your mana. Also make sure that you have excellent map awareness. It will only benefit you because of his global ultimate. If you get level 6 and you are still bottom lane with your support, check the other lanes occasionally to see if any enemies are low health so that you can ult them and get the kill. But make sure you remember how much damage your ultimate does because you don't want to waste it on someone that will have 30HP left after your ult. It has a very long cooldown, so only use it if you are sure you can get the kill. Another situation you can use your ult is if the team is in a battle and you are pushing a lane no where near them, and cant get there in time to help them, using your ult will reduce all of their health and help your team finish them off and give you the assists.
Basically guys Karthus support is OP and you will carry many games. I hope this guide helped you. I have been playing Karthus for many years and support I feel that he may actually shine more than in mid. Give it a try and leave any questions or comments below.