Hello everyone, I am an avid Karthus player who has acquired a wealth of knowledge on this champion. In this guide I will introduce several builds and speak mainly about how to lane him and turn him into the scary lategame "hypercarry" nobody likes dealing with.

PLEASE READ THE FOLLOWING
This guide is a work in progress and I wish to make it the most comprehensive and detailed available. So please leave a comment or contact me about something you wish to have added. I will be adding more and more matchups as well as videos on farming and how to win certain engages etc.

Pros:
He deals one of the highest amounts of sustained damage in the game
His Q late game can deal more than 800 single target damage
His ult late game can deal over 1000 damage to each target
He farms very well in lane and under tower

Cons:
He is quite squishy
If you do not land Q's you do not win engages
If you do not know how to play him well you will need to play safe
His ult channel can be cancelled by a lot of champions
It is difficult to know what to do and when to do it in many situations.

You need to pick flash on Karthus!!! I cannot stress this enough. It will allow you to 1. Survive ganks that you normally couldn't and 2. Flash engage on the entire enemy team to output your damage. There is no questioning this, you need to take it.

There are two other summoners I consider taking on Karthus. The first is exhaust and personally, this is an amazing disengage spell against assassins but it is also a great spell to lay down a significant amount of damage. This will let you beat your opponents in engages and help you win lane. I strongly urge you to consider taking this summoner. Furthermore, this will reduce their damage output by 50%, so if they have a burst combo you can use exhaust on them during the middle of it to reduce their damage significantly.

The second summoner I have tried using is heal/barrier. Personally I like heal more than barrier because 1. it is health that you keep and it will allow you to stay in lane a little longer and 2. it will boost your speed for a short period of time. This will allow you to survive ganks and also help you win trades, however, mathematically speaking exhaust does increase your chances of winning trades more as it reduces the enemies damage by 50% for 2.5 seconds as well as slowing them to increase your chances of landing Q's.

tl;dr TAKE EXHAUST AND FLASH

This is the most important aspect of Karthus - everyone who has ever played league of legends will tell you that YOU MUST FARM AS KARTHUS AND DO IT WELL. Now, unlike many guides you will see here, you should not spam q all game and be the mid laners ***** until you get to late game. You can farm, harass and even kill superbly. Farming, though, will require finesse and practice. Most of the time your single target Q damage will be able to kill the minion, but the multi-target damage won't - by placing your q in the correct spot you can execute that minion rather than pressing q in the middle of 4 and missing your important CS.

If they want to push, let them. Allow the tower to attack the melee minion twice then use Q on it. For ranged minions, let the tower attack it once then use a single target Q to kill it. Rinse and repeat whenever they push. If you accidentally use a Q and it hits multiple targets, autoattack the target and turn on your E to kill it. It is costly, but you need to farm!

A little trick - if you are trying to push out lane before backing, activate your E until the minions are almost dead then spam Q. You will most likely kill the minions with the multi-target Q damage but you will regain mana from this, rather than losing it from your E. This may be vital because you could be required to ult right before you back or fend off a gank with your wall. The little things count.

Note I will be including several videos later on about how to farm under tower and in lane effectively.

You should use the following techniques to play Karthus to his maximum potential.

1. Karthus' Q and W will give vision, yes, give vision. ALWAYS scout bushes with your Q so that you know they are safe. When laning always always always Q the bush if you are worried that there is an enemy waiting in there. Keep in mind that, even if the Q doesn't give vision of them, it might very well make them think it is warded and they might back off. The ends of your wall will also give vision but never use it unless you are desperate. More often than not there will be someone, or a lot of people waiting in the bush. If your wall gives vision of them but does not slow those in the bush, you will be engaged upon and will have a miserable time. If you use your Q and find someone in the bush, back off and if they follow, wall and run.

2. For god's sake never use your E to farm unless you have blue and a tear. It depletes mana and won't deal as much damage as your Q, besides, you want the wave to push to you, you don't want to push the wave. Whenver you engage opponents that are low level, never activate you defile unless you know the fight requires it, why? If you activate defile and spam Q after you use your wall you will have no mana left when you need it to farm. You will likely be engaged upon and won't have enough mana to scare them off or defend yourself. Keep this in mind. As you play Karthus more and more you will know when to activate defile in engages and when not to - this is important because it will determine how well your laning phase will go.

3. If some tricky bastard is on low HP tries to kite you and you are low HP and are low-ish on mana, do not spam Q and die like a welp because you couldn't land them, activate your defile, follow and auto-attack them. This will GUARANTEE a kill, and removes risk of dying from your situation. Likewise, if you are confident you can land your Q's on the enemy, deactivate defile and spam Q like no tomorrow. It will be a lot more risky but it will get the job done. Keep in mind that the higher ELO you are, you will need to land your wall to get these Q's to hit, and if it is on cooldown, activating defile instead might be the better choice.

4. There is no suicide Karthus. Lategame your defile will do 300 damage PER SECOND. If your team is with you and you see enemies bunched together ,activate wall your E and flash in for the love of god. You could , instead, spam Q's and try to walk in amongst them, but you will likely get CC'd and die, allowing the enemy to disengage. You need to be in the middle of the team and you need to do damage for as long as possible. This means you pay very close attention to any skill shots and you need to land your Q on priority targets.

The best Karthus' will be able to proc single target Q's off while being able to deal significant amounts of AOE damage for as long as possible. DO NOT flash, then stick try to stick to the adc for as long as possible and deal barely any damage to anybody else, the adc can kite you and you will likely die outside of their entire team, meaning your massive AOE damage will go to waste. Try to get as many people as possible in your E while laying waste to priority targets. Activate your seraph's embrace and watch your health carefully, right before you die activate zhonyas. When you die ult. If you execute these things in order you will live as long as possible while dealing the most damage possible for your team.

5. If you buy will of the ancients for some reason you MUST always use your Q on single targets if you want the maximum amount of lifesteal. Why? I did the math, unless you are hitting more than 6 targets with one Q you will get less health back than if you were hitting a single target. Keep this in mind if you buy WOTA for jungling or if you want to mess around a little

Yes, this is a chapter because I cry every time I see a Karthus ult in the middle of his lane when he shares it with someone who has CC. I originally began listing all the champion you should never use your ult in the middle of the lane with but the list was, honestly, too long. Most mid laners have some CC that will stop your ult channel and if they don't they are highly capable of bursting you down before you ult can finish channeling. Whenever you are planning on using your ult to secure a kill walk behind your turret if your lane is pushed or well behind your turret if the lane is pushed into your tower and immediately hover your cursor your ult.

At level 6-11 it should be do 250 base with around 150 extra damage MAXIMUM (this is around level 10) if your enemy has 240 health and your ult does 260 damage, most of the time I would ult but it wouldn't work. Their magic resistance will account for a little extra damage absorption. Usually I would ult if the enemy has 30 health less than the damage my ult will do. I CANNOT STRESS THIS ENOUGH, NEVER ULT TO GET A KILL IF
A. if the kill is obviously secured and
B. If there is ANY doubt that they will get away. You must be confident about pressing R, if you don't make a decision fast enough, they will regen too much health for your ult to kill them, I am not kidding.

After laning phase you need to recognise when to use your ult: before a massive team fight or after you have died. Most of the time I use it after I have died, why? No interruption and guaranteed damage. If you use it before you die you could A. Die and your ult will cancel B. have the other team disengage, heal, then force a fight before your ult has come off cooldown and you might very well lose that fight. ONLY EVER ULT BEFORE A FIGHT UNDER THE FOLLOWING CIRCUMSTANCES.
1. You are too far away from the team fight for some reason and your team has been engaged upon
2. The enemy team is HEAVILY INVESTED IN TAKING AN OBJECTIVE i.e. baron and dragon and you know you cannot get into the middle of the fight or you are worried your team may not win the fight.

These are the only reasons why I ult before a team fight. It is important to know the difference. If you ult after the team fight under the wrong circumstances you may not be able to kill everyone you need to and the enemies will push objectives.

If you are playing ranked I do encourage you not to first pick Karthus. He can win if played well but, unless you are a seasoned Karthus player, I would not first pick him in case they decide to make your life a misery and pick an assassin you decided not to ban.

Ahri

Difficulty:
Pre level 6 - 2/5
Post level 6 - 3/6

You can win this matchup if you play well. Pre 6 she has no innate "escape". If you have warded well and Ahri misses her charm immediately use your wall on her and spam q making sure you will deal single target damage to her. After this back off and continue farming. Post 6 ALWAYS make sure her charm misses if you want to harass, otherwise you will get combo'd and you will most likely die. Always ensure you either dodge her Q or only get hit with it when it goes through you originally. Against Ahri I use my standard build, making sure I rush catalyst and boots.

Annie:

Difficulty:
pre level 6 - 3/5
post level 6 - 4/5

This match up is moderately difficult. Annie has very high AP ratio's and, although she has no escape, her stun will allow her to deal immense damage then back off. Pre level 6 Annie will most likely max her Q. ALWAYS ALWAYS ALWAYS be aware of how many "stacks" she has on her stun. If it is less than 2 and she has just used her Q you can wall, lay waste then back off. If she has 3 charges she can Q, and while her incinerate is in the air, she can shield and proc the stun from her Q still - not fun. What does this mean? If she has 3 stacks, play it fairly safe.

Post 6 you will have an even more miserable time if she plays well. She is a 100-0 champion. I recommend rushing catalyst into zhonyas. If she plays poorly, she will q to stun you then use her 100-0 combo. If this happens and you are scared you might die Zhonyas right before the Q hits and you have a chance that she will blow tibbers on you while you are invulnerable.

Le blanc

Pre level 6 - 3/5
Post level 6 - 4/5

The famed matchup - master sustained damage vs master burst damage. Believe it or not, even before she could silence you you could win this. Farm normally, and when she uses her Q, you should be able to avoid being hit with her W by playing safe until her Q wears off. If she engages with her W then re-activates it immediately (and if you aren't effected by her Q debuff) immediately punish her by placing your wall then spamming Q. Remember, she will still try to combo you even if she is quite low health. I don't usually recommend this, but, if she does try to combo you while she is low, immediately turn on your E once she uses her W on you. If she does not re-activate distortion, spam Q, if she does re-activate distortion, wall her, spam Q while your defile is active.

Post level 6 is where this gets tricky. Normally your sustained damage would work, but she can use her distortion again to avoid your wall and Q damage "combo". Post 6 this matchup will become a nightmare if you do not play relatively safe. Keep in mind that, for the most part, le blanc needs kills, so she will put herself in some awkward positions to get some. Recognise these positions and punish her whenever you can. I rush catalyst into zhonyas; however, this is one of the very few times I will make an exception for rushing a giants belt into rylia's - this will allow for more immediate "beefiness" if things aren't going your way in lane.

Nidalee

Difficulty - 3/5

Personally I love this match up. She can try to harass you but you can stay behind your minions for the most part. Most Nidalee players will try to "flank you" i.e. stay to the extreme right or left of lane to land spears, thankfully, pre 6, we can punish this by using wall, then spamming q. Because they are on the extreme sides of the lane, you will get single target damage. You don't NEED to do this, you can just farm very efficiently against her instead, but you need to recognise that you can harass her and farm efficiently at the same time.

Nidalee's cougar farm can be petrifying, however, if you remains behind your minions and she leeps towards you, immediately activate W then spam Q. IF for some reason, your W misses (cuz u bad) activate defile, Q and try to disengage if you are seeing that she will output as much damage as you. Why? If she deals as much damage as you and you go ham to the point where you are both nearly dead, she will just heal it all back up, re-engage and win lane from there. If she engages in cougar form will usually jump on you, meaning she has activated her escape to get in range, she will be able to take your harass until it comes off cooldown. Due to her need to be in melee range, you should be able to land a few Q's without needing to wall, otherwise her pathing will become staggered and she won't be able to reach you. You should always win this match up.

Soraka

Difficulty
pre level 6 - 4/5
Post level 6 - 5/5

Cry. But seriously, aside from a few select champions this will be your worst match-up. Most people will say "Soraka counters Karthus because of her ultimate" No, it is never that. She needs to be close-ish to you to deal damage, however, when she gets close she can silence you and spam Q like you, except she can leave the fight and heal it back up. In this matchup I avoid conflict if at all possible. I don't like being her *****, but if you fight and it looks like you are winning, she can heal, silence, disengage and fight you again within the next minute but she will have more health. And I lied, her ult, despite it being weaker, will almost always cancel your ult. For this match-up I rush a razor blade and a bottle of sleeping pills...but seriously, not much you can do other than use the standard build with slight variations depending on whether or not your getting wrecked really hard.

Galio

Pre level 6 - 2/5
Post level 6 - 3/5

Galio is an underrated champion who can deal a surprising amount of burst damage with his Q E combo. In lane you should be able to farm very effectively but keep in mind you want to be rushing boots to avoid his Q damage. He needs to be in melee range so follow my prior advice and always try to proc a single target Q on the minion it looks like he will be attacking. When you two engage pre level 6, try to wall AFTER he uses his shield. Your wall reduces % magic %%%%%%%%%%%%%%%% magic resist, meaning his magic resist will be decreased by a significantly larger amount if you wall after his shield is activated. This way you can wall, spam Q, and get some damage off then go back to farming.

You should always beat him in farm if you harass effectively. Keep in mind that a smart galio will use his shield well: if you are both quite low he will shield and gain aggro from your minions - THIS WILL HEAL HIM. Punish him for this significantly. by placing your wall, spamming Q. Post level 6 is where this get interesting. Play it safer than usual, but try to push your lane, he farms horribly under tower and, if you get too close to his tower, he will flash and ult you. You will gain tower aggro, take the full damage of his ult and take his combo, which is almost a 100-0 combo. For this match up I use my standard build, making sure to rush RoA for the extra beefiness it gives.

Katarina:

Pre and post level 6 - 4/5

I hate laning against katarina but that is simply a person qualm. Theoretically you should do okay against her. Whenver she goes to Cs with an autoattack punish her by pressing Q. Farming should be okay, however, she can spam Q on you all day and you lack sustain. This may be one of the VERY FEW cases that I would take spellvamp quints. Pre 6 she can be a bit of a nightmare, she will usually start boots, meaning, even if you wall and spam Q, she can dodge them. If the Katarina you are versing for some ungodly reason wastes her E to get to you, Wall IMMEDIATELY, then walk back a tiny bit before turning on your E and spamming Q. Why walk back? If she is close to you, her W (the spinning thing) gives her movement speed, this will allow her to keep hurting you while dodging your Q's.

Post level 6 she becomes a bit of a nightmare and I always rush catalyst into zhonyas. If it is obvious she will combo you, zhonyas, then combo her. Keep in mind she is a very tricky champion to play against and you will require ganks. She has no CC but for the love of god never ult in lane, she will 100-0 you, laugh, walk away.

More often then not Katarina players will get bored of playing against you and gank other lanes repeatedly. You should follow IF YOU ARE AHEAD AND HAVE A CS ADVANTAGE ONLY. If you are not ahead, farm and repeatedly warn lanes about her possibly ganking their lanes. If you follow and you are behind, you will die. A very important thing about playing Karthus is realising that your teammates are probably stupid and that you should never listen to them about following other champions when they leave lane unless a big fight is about to emerge. Use your own judgement.

Kayle

Difficulty
Pre level 6 - 3/5
Post level 6 - 3/5 to 4/5

The bane of my existence. Another lane I hate playing against in general. Pre level 6 you should do okay, keep in mind that you should stay the maximum distance away from her when using your Q, otherwise you will be slowed and take significant damage from her righteous fury (her E). I farm normally in this lane and ask for ganks whenever I can. Keep in mind that she has no escapes, but she does have a heal which will allow her to sustain in lane a little bit better than you. Pre-6 this matchup can be in your favour if you recognise that she will occasionally last hit with an auto-attack not empowered by her E. Punish her by using the wall Q combo. She will most likely push your lane because her E is an AOE damage attack. You farm extremely well under tower and should let her push you into it. Free farm for you and no inherent danger.

Post level 6 if you do engage and almost kill her MAKE SURE YOU MAKE HER USE HER ULT AND THAT HAS LITTLE TO NO MANA. If she used her ult, and she has mana, it is likely that she will heal before you ult and possible survive. Make sure of the following after engaging
1. She used her heal towards the end of the fight and she has quite lo health regardless: level 6-11 around 100-150. If it is obvious her heal is on cooldown 200 hp is acceptable.
2. She has used her ult. For gods sake do not engage, laugh, walk away, ult then realise she didn't use her ult. She will laugh at you and you have wasted your ult on nothing.

Please note kayle is an absolute nightmare lategame if she is fed so ganks should be required throughout laning phase to get an advantage.

Fizz

Difficulty
Pre-6 2/5
Post 6 3/5-4/5.

His troll poll will test your resolve. Pre level 6 he will most likely Dash to you and deal some damage. When you wall he will most likely playful trickster away and begin farming again. NOTE THIS. If your wall slowed him before he playful/trickstered away, you will have slowed him and reduced his magic resist. At this point spam Q but DO not walk up into defile range and activate it, he will most likely beat you unless you have a lot more health than him. Pre-6 I combine this strategy with the pre-discussed harassing strategy against melee mid laners. You should win this match up pre-6. But be careful, he does a lot of damage in a very short time. Post level 6 is where this gets...interesting.

If he engages you with ult and you are both full health, I wall, heal and try to back off, landing Q's if I know I'm outside of his dash range. He can playful trickster over your wall if he is good - I advise you to still not engage if you know you can live. If you know you will die I would engage and land as many q's as possible. Keep this in mind though, if he does not use playful trickster again during your engage, DO NOT ULT...DO NOT ULT. It will most definitely be off cooldown and you would have wasted your ult. If he uses playful/trickster again during your engage and you know your ult will kill him, most definitely use it and you will secure a kill.

In this matchup I rush catalyst into either rylias or zhonyas, preferably zhonyas. By doing this, he will ult you and do the following - Dash, or playful trickster to you. No matter what he does, attempt to zhonya's mid animation to negate the damage of the ult and his next ability. When you come out of zhonyas you should activate your E immediately and place your wall. If he playful/tricksters away, you should keep defile active if you have adequate mana then chase him down and spam Q. Keep in mind not to go too ham, he will most likely attempt to re-engage, at which point you should back off with defile active and spam Q's to distort his pathing so you can get away, or punish him for walking straight towards you which could net you a kill.

Talon

Difficulty
Pre level 6 - 4/5
Post level 6 -5/5

This is a god-awful lane if I've ever been in one. You should be able to harass him with your Q's when he goes to take cs but make sure that you do not get too close. otherwise he will use his rake to harass you, or worse, use his pre-6 combo. If you farm safely behind you minions or even amongst them and he silences you. Immediately use your wall and walk backwards while spamming your Q on him. You don't want to stay in melee range for very long otherwise he can be quite deadly. Keep in mind he will go ham whenever possible because he is an assassin. If you have pushed the lane too far and he goes ham on you, wall and do not use Q if it looks like you will lose the engage. Q has a cast time and it will take longer for you from getting back to a safe position.

Post level 6 he becomes a nightmare. Much like you in certain matchups, he can use his combo without needing to use his ult to do a fair amount of damage to you. Keep in mind that at level 6, if he silences you, immediately turn on your E and DO NOT IMMEDIATELY WALL. If it looks like he can kill you immediately exhaust him and spam Q's while walking towards your tower. This will slow him if he decides to ult. If he ults while exhausted, place the wall on yourself then walk towards tower. He will not be able to get behind you to activate his ult damage. You can go toe to toe with him if and only if you harass him while he tries to take CS. Rush boots, catalyst and/or seekers arm guard.

Kassadin:

Difficulty
pre level 6 - 2/5
post level 6 - 5/5

The famous antimage. You can win, but you need to keep in mind that he is still somewhat strong pre 6. Most people will read a guide and think they should go ham pre-6 when in fact he can out-dual people if you play poorly. Harass him when he tries to Cs and make sure not to be in melee range, it is a big mistake. In order to beat him you need to wall him occasionally when he goes to Cs, then use lay waste to wittle him down. He will max his Q against you and this will negate a significant portion of your damage, but you will still do damage. You win this match-up pre-6 even without jungle help. A key thing to note is that, if he farms with his Q, you must IMMEDIATELY PUNISH HIM. Why? His Q negates magic damage and as such you can outdual him if he has it on cooldown.

Also, and I cannot stress this enough, if he uses his E and Q then you have to engage him. His E does a significant amount of damage and requires people to use spells for it to charge. The problem is, because your Q is on a small cooldown, it will always be up, but you should know that he needs to be punished if his E and Q are on cooldown, otherwise you will not win this lane. Post level 6 the nightmare begins...

Post level 6 kassadin will riftwalk to you, Q, W, E and autoattack. What does this do? It will put him in melee range and it will negate any of your initial magic damage. Immediately wall, turn on your E and spam Q. Your positioning should put him in a single target damage zone. When walling, move backwards immediately to get out of his auto-atack range. You will most likely lose this engage and that is okay. You need to scare him off though and this will do that. Now, always rush a catalyst into RoA in this case, I cannot stress that enough. This will give you some kind of "sustain" against his repeated engages onto you. Ask for jungle help even if he is post 6. He will engage on you whenever he can and it will be scary, but if your jungle pays attention they can CC him and you should be able to net a kill.

VERY IMPORTANT THING. His Q will interrupt your ult, so never ever ever ever ult while he is ANYWHERE near you. If you ult in the middle of the lane, he can Q you, then riftwalk away. Also it is extremely important to keep in mind how whether or not his Q is on cooldown. If you ult him and he is still in lane, he will simply Q a minion and block the majority of your ultimate's damage then walk away.

Zed

Difficulty
Pre level 6 - 3/5
Post level 6 - 4/5

Pre level 6 ALWAYS try to harass him whenever he goes to farm. He will constantly try to land his Q on you and at first it won't hurt, but once he is around level 6 it will. Pre level 6 always make sure he has used his shadow and it is on cooldown before placing a wall. You don't want to use it while his shadow is up because he will re-activate it and walk up to you, auto-attack and chunk your health. You can go on par with him pre 6 with your q's, but he has way more sustain than you, so it is best not to. Ideally, he will use his shadow and Q and not re-activate it. Immediately wall and spam Q. do not activate your E and walk up to him because of the aforementioned reason. Also RUSH A ZHONYAS AND BOOTS.

Post level 6 he can hurt a lot. When you see he is ulting you, and if he has enough health to survive you, wall yourself while he is ulting and walk away. If you think you can survive just keep walking and juking his Q. If it is uncertain, use your heal or exhaust to get away. If you know you are dead, then isolate him and get as much damage off on him as possible before you die, you might kill him. Always keep this in mind - if you zhonyas too early his ult will deal damage - not a lot, but it will still deal damage. Wait for a short time before activating it to reduce his damage and it's. You can win this match-up but lategame he will consistently ult you and you will probably activate your zhonyas before you want to in a large team fight.

Rush seekers into zhonyas, but always try to get some health in the form of either a catalyst or a giants belt - armour is nothing without health.

I hope you read through the chapters in my guide and your Karthus play improves because of it. If you wish to contact me or leave a comment do so, I will not hesitate to respond and/or act on your request by adding something extra to the guide.