This guide is written for those who get stressed by solo ranked and get thrown to low elo (although i recommend to get ranked team since it will be more fun and more reliable :P). but still i will present this for you guys who wanted to go out of low elo, karth is the right choice for you

Pros
-Press that R BUTTON!!!!!
-Highest damage late game (if use properly)
-Good Farmer
-Can just walk to kill enemies champ
-Have i told you that you can press R to kill your enemy champs?
-

Cons
-Squisy
-easy to learn hard to master
-need to get enemy close to him to deal massive damage
-need a lot of farm to get to its full potential

The first mastery is for taking ROA, and the second one is for taking AA, but it also work the visa verse. it's all personal preference

first of all. i will say these are the item i usually if not always get.
its all of your preferences but i just think that this is the best item for my playstyle.
the first build is more defensive build and the second one is for more agresive play

explanation

first build :
this build is for fighting against better liner like malz, brand or morgana ( someone stronger than you early)
taking boots, 3 pot works well with movement speed buff from the masteries
then rushing catalyst for lane sustain.
In this build you dont get AA so abysal specter feels better than banshee


the second build :
this build focusing to get tear ASAP to fully charge it
Since we already got AA i prefer taking banshee instead of abyssal

remember this is just how i usually play, you can change it to your own condition

here a few reccomendation


take this if you need some spell vamp


4 ad champ? abuse it


too much cc? this will be good


you never die? grab this


need some slow? take this, but only take it as the sixth item


give you massive hp, but only take it as sixth item


feel free to grab one of these anytime above 25+

This is in my opinion the best skill sequence for karthus


your main early damaging, a very good harassing and farming skill if you doing it correctly. take it every time you can


This is your deadly AOE spell, with is passive and an extra laning sustain. max it after


this is your only cc, take 1 at lvl 4 so you can gank and anticipated being ganked, max it last


PRESS THAT R BUTTON!!! need more explanation?

I take revive and teleport, WHY? this can give you a ton of time to fight late game, where you are at your best. but if you have chosen this spell. you have to know when to use it and when not to use it.

another viable spell


The best summoner spell for positioning IMO, but it only draw back is its long cooldown


You are slow in the beginning, so extra movement speed wont hurt you


only use it if you have master karthus, mastering it not good at


good for extra cc or taking care of that AD champ


the extra AP is nice


Too much cc?

Early game
this is the most risky game phase, if you get strong enemy in mid like kass, le blanc or malz just farm with and ask for a gank.
if you are taking the boots first you can have a more safe line, but if you take mana crystal first, you got to be very aware of gank

Mid game
this is when you got to start some ganking, see dont use your to ks your friend, use it to assist them in the beginning of the fight, even if you are starting to gank . you still have to farm as many as you can

late game
this is when you shine the most, you should have almost complete 6 item, with this you can jump inside a team fight, use all your skill, getting focused, turn on zhonya, die. then use , use your revive and teleport only if you know that it will win the team fight. remember, never go in a fight when you know you will lose it, it may cost you a game

in the end, i still believe that karthus is one of the best carry in the game, because of his presence in teamfight and his deadly spells. thank you for reading my guide. i will constantly updating it and add more element that needed. remember, play as you want. because everyone have their own playingstyle