Hello, I am Karthus man (not my lol name). I am a Karthus main and have peaked at around a 70 percent win rate with him. I am not playing ranked at the moment, because of the new season, but I probably will once the new season starts. I hope I can teach you everything Karthus. I will primarily keep my guide section in the notes section, and if you have any questions, comment down below.

ENJOY!

As karthus, you always want to start blue buff because karthus has crazy mana problems in the early game. You then want to get gromp and continue clearing, taking everything except krugs. The reason you don't take krugs is because you want to take a scuttle krab, which should spawn like 10 seconds after you finish taking red. In the early game, you don't want to fight at scuttle krab, unless you get a lot of help fighting. Pretty much any jungler will beat you in a fight early game (unless you are really good or they suck)so you should just go to the other scuddle. After that, you should either go clear more,or gank a very gankable lane, but be weary, never gank any champ if YOU are very low, because there is a chance that you don't get enough health and die or have to burn flash.

Mid Game, you should be a fair bit stronger. Keep farming and taking objectives and group for team fights and you will get to late game faster. The more kills and CS you get, the faster you will get to late game.

Late game, you should be really strong. You should be able to practically 3 shot people with your Qs. Stay back in teamfights and deal tons of damage with your Qs and then go in at the end and clean up. Try to save your ult when you really really need it and don't use it to steal kills, but it is fine to use it to steal kills from your allies early game ;).

Ulting is one of the harder things to use on karthus. His base ult is 200 dmg, which sounds like a lot early game, but magic resistance and other things makes it hard to tell if it is going to secure a kill early game. One thing I like to do is if you aren't certain about your ult securing a kill when your ult is LVL 1, don't take the chance and waste your ult unless the person has a big shutdown about 300+ gold. Once your ult is level 2, it is a lot easier to see if your ult will do enough damage because by then, you have ap items and the base dmg is 350, so if you ult, it should be at least 475 dmg. Late game, your ult does 500 base damage, and plus all of your ap items and other things, it should do between 800 and 1000 damage. But again, by then if there is a tank, they probably have a bunch of armor and magic resist, so it will probably do less the 600 dmg.