Karthus is a mage who can inflict enormous damage over time and all over the map, which is compensated by a lack of mobility. AP and magical penetration are the stats you need.
Sorcerer's Shoes is a good choice. Once again, any magical penetration available is good to take.
- First of all, get your jungle AP Stalker's Blade - Runic Echoes jungle item as soon as possible, it's a very good item and an amazing powerspike.
- Morellonomicon gives you more AP and magical penetration. With R - Requiem, you can also cut the regeneration of an enemy on the other side of the map, useful if your allies are in the middle of a battle.
- Rabadon's Deathcap will bring even more AP to your build.
- If you feel the need, set out on a Zhonya's Hourglass. E - Defile continues to inflict damage around you while in Stasis.
- If your early game start is favourable, don't hesitate to buy a Black Seal (and why not upgrade it to Mejai Soul Thief afterwards).
- Void Staff will give you even more magical penetration. This will increase your damage on tanks and inflict raw damage on the most fragile targets.
- Speaking of tanks, if the enemy's composition is full of them, Liandry's Torment can be very useful for you. In addition, W - Wall of Pain will increase the passive damage of the item if an enemy passes through it.
- Spellbinder is in the same spirit as Rabadon's Deathcap: to gain a lot of AP. Use its active at the right time to maximize your damage.
Karthus has several big strengths, but he also has some shortcomings... His jungle cleaning is one of the fastest in the game, but since you have no escape or control spells, be very careful of enemy junglers and enemy laners that disappear from their lanes.
- As Karthus, you must get your level 6 as soon as possible, then your jungle item (Runic Echoes). From that moment on, take a good look at the map and use R - Requiem when you think it will be lethal to one or more enemies.
- In combat, death must not scare you! In a 5v5, try to dive into the heart of the enemy team. Once dead, you have 7 seconds during which you can continue to cast your spells without worrying about the controls and your remaining mana.
- Level 1: Take Q - Lay Waste and do your blue buff
- Level 2: With your E - defile clean your Wolves and Gromps.
- Level 3: if you want to gank mid before going to the red side of the jungle, take W - Wall of Pain, if not, level up your damage and clean your remaining camps before going back to base.
- level 6: You know have a global pressure, continue farming as much as you can. You're also able to solo a drake with enough mana or a blue buff.
- Note: You may also start with your red buff if it's benificial for your laners.
Tips & Tricks
- Q - Lay waste deals double damage when the spell only hits a single target; use this to maximise your jungle clean as well as your damage to enemy champions.
- Q - Lay Waste and W - Wall of Pain both give vision when used. Use them to check bushes where you don't have a ward.
- W - Wall of Pain reduces the magical resistance of enemies passing through it.
- When your passive is up use your ultimate spell soon enough or you won't have time. You have time to use at least three times Q - Lay Waste then R - Requiem