I am elosanta, I stream on Twitch.tv/elosanta I have participated in the Baltic esports league and I have peaked rank 8 challenger 929 LP at the start of season 9 and one of the champions I used to climb was karthus, I feel like he is still strong up to this day, and I have many games on him so I think I'm qualified to write a guide.

As many of you know, Karthus is usually played in the jungle, more rarely Mid/Top. I started playing Karthus botlane after the infamous s8 ADC nerf when infinity edge became basically useless and everyone was playing mages / tanks botlane. Even to ADC's being restored their power with crit items right now, I just feel like Karthus botlane completely STOMPS every single lane down bot. The only lanes that I have been consistantly losing against are Syndra or some other mage botlane. I am practising many other mages botlane (Swain and Syndra seem to be my favorites now, but Heimerdinger and Veigar seem to be really good at this moment aswell). The only downside to playing a mage botlane is that your team will usually have too much AP power, which is why I usually ask my team to pick as many AD champions as they can Mid/Top/Jungle. Even if all three go for AD champions, Karthus just has so much damage that its enough to supply the whole team with AP damage even though 80% of your team is AD based champions.

On your first recall most of the time you want to buy Sorcerer's Shoes, because they give you the ability to hit more Q's as you have higher move speed and you have more base damage as well.

Zhonyas was a very bad item to get on Karthus in season 10, but in preseason and season 11 you can combo it very well with the burning build (liandrys) but in this build I am showing the liandrys + malignance build so Zhonyas should be a bad purchase.

Build will be always updated in the basic setup, so refer to that, since it changes patch to patch and it takes a lot of effort looking over every chapter to keep it updated, so the general build will always be correct!

In the laning phase you want to play aggressively and outpush the enemy lane into their turret, this lets you poke them under their turret making them lose health and they will have to give up minions as well. If doing this make sure to pay attention to the minimap as you will get ganked a lot while pushing. This is great for solo carrying games because if the enemy jungler is ganking you he is not ganking your teammates, which can get them ahead and as long as you don't die from the ganks you should be way more ahead than your enemies as a team.

Edit of season 14: I have tried builds with liandrys and Malignance and I can say by far the strongest build is with malignance and liandry. You can try going liandry first but I have noticed I don't like it as much as you run out of mana really quickly. The build listed above will be up to date patch to patch.