Karthus is an overall strong champion that has many styles of play. He doesn't really fit in the current meta of strong pressure assassins that can easily get kills in lane, however once he reaches level 6 , he applies a ton of pressure all across the map. I believe that Karthus is high skill-cap champion because his damage potential with his lay waste is insane if you're able to land his lay waste consistently.
Lay Waste - Max it always because its the majority of your damage. Wall of Pain - You usually max this ability last, however it really depends on that composition you're playing against. You want to max this spell if you're playing against a team that engages on you and you want to kite back. Otherwise you would max e. Defile - Max this spell 2nd if you have enough CC and you want to go full damage and dive in, you want to max E when your team is an engage/dive team. Requiem - Obviously spell into this whenever you can, it provides insane global pressure and helps your teammates win lane.
THINGS YOU MUST KNOW AS KARTHUS TO BE SUCCESSFUL!!!!THINGS YOU MUST KNOW AS KARTHUS TO BE SUCCESSFUL!!!! 1. You want to always push your lane as fast as possible to prevent the opposing mid laner from roaming. Pushing immediately is insanely important because it forces your enemy to lose farm. You never ever want to let your enemy roam the map for free. Roaming means that you think you'll be able to get an objective/ kills out of your roam, but by shoving your lane and forcing the opposing mid laner means they'll fall behind if their roam is not a success. 2.Karthus has the disadvantage against most if not all match-ups against assassins because his killing potential is must less . 3. You generally do much more later in the game. 4. Don't feed especially against assassins. They will crush you. It gets easier and easier for them to kill you and kill your teammates.
For Mid laners, the game is all about applying pressure and helping your teammates get objectives. Karthus does not really fit in this current meta because high pressure champions that roam for objectives are much more useful. Karthus lacks mobility, so it is very hard for him to roam and pressure objectives . It is also very easy to catch him off when he is warding/roaming . In order to play karthus in this current meta, you have to really focus on only using your ultimate to help your teammates and your sole objective should be farming and doing more later in the game. Karthus is an insane champion late game, but his pressure early game is garbage.
As a Karthus player, your sole purpose is to farm, carry late game, and apply pressure globally with your ultimate. Karthus does not have an escape, so you have to be very cautious when farming. Karthus does massive damage early game with his lay waste so ganks are always possible when your opponent is out of position. If you time your wall of pain correctly, you can easily burst an opponent down with jungle support. If you're a new karthus player, it will take a couple games to get use to the range of karthus' spells on smart cast.
Karthus is fairly strong mid game, after hes farmed his [img=items/sorcerers-shoes.png][img=items/catalyst-the-protector.png]. Mid game is when you start to fight for objectives and it is very important to get blue on karthus. After the early phase, karthus becomes stronger and stronger and stronger. It is important for you to always get your blue buff because it allows you farm wraiths,wolves and clear mid lane instantly.
There are two ways to fight team fights with these summoners([imgsmall=summoners/exhaust.png][imgsmall=summoners/flash.png]); engage or disengage. You must be able differentiate between each and every individual composition. Sometimes it is better to kite and hit the closest target with lay waste, while exhausting the enemies that charge in. However in most cases, you can just flash in and exhaust the AD carry and try to do as much damage as possible. It is almost always better to focus the AD carry, unless their AP carry is fed. You generally want to fully engage on a composition that has sustain damage that doesnt have much aoe. You want to kite around, when the enemy has high burst ... where they can easily instantly kill you.