So. Karthus. I imagine 90% of the people reading this guide are looking for a quick fix to a build for a few playthroughs of a lulz trollchamp during freeweek. Another 9% are sincerely looking for a way to play Karthus, but have no fucking idea what they are getting into and will quit Karth. Finally there is 1% who also have no fucking idea what they are getting into but will like Karthus and keep playing with him no matter how low their KD gets each game. That 1% is what I hope I'm reaching.
This guide will not teach you how to be a good Karthus player. That's what hundreds of games with him will teach you. Instead, this guide will simply show you how to play Karthus, the way I played him for 400+ games.
A big WARNING: Make SURE that you are summoner level 30 or almost there. Karthus is not for inexperienced players. Even if you roll with him from 1-28 you will get your shit raped backwards once you reach higher elo and people start to actually DODGE your spells instead of running backwards in a straight line.
So, onto the guide!
Whatever the hell version it is
I've pretty much stopped playing LoL- maybe I'll give it another go once Domination or whatever it's called comes out. I'll fix the red text in the guide sometime. It's behaving very oddly (I fixed the Updates section but I can't fix the other ones). JESUS CHRIST MY GREEN TEXT IS GONE TOO. Yeah, guess if you're new to reading my guide, you're going to have to use your imagination.
Apparently all my red text- the important stuff that I emphasized- is no longer red. I'm rather confused.
Not much here- just keeping the guide on the top of the page, and mentioning how Karthus hasn't been changed in the Updates section. Might add some more "Champs you may meet in mid" later.
-added pictures :D
-changed the color of some text so it isn't so boring
-fixed up some stuff
-added Exhaust Baiting section
-added Updates section (also known as the "the hell are you doing Riot?!" section"
| Death Defied
| 8 seconds to do as much as you can after you die. In an ideal game, this passive will be basically USELESS because you won't be dying. I will be going over this passive IN DEPTH later in the guide.
| Lay Waste
| A 1 second CD spammable aoe mini nuke that does twice as much damage if it hits only one target.
Most players' reactions to this spell: "OMG I HATE THAT ANNOYING BOMB IT ALWAYS HURTS ME WHEN I'M FARMING!"
A player who has played a good Karthus's reaction to this spell: "Karthus needs to be nerfedddd!"
Most Karthus's reactions to this spell: "I need to predict my target's movement 24/7?! Fuck."
skill allows Karthus to reach massive amounts of magic dps. But if he misses one or two he loses out to other nukers in total magic damage. Think about it. Annie's full rank 5 Combo doles out 1035 base magic damage. Karthus deals 240 base magic damage with Lay Waste. If Karthus pulls off all his Lay Wastes (single targeted on enemy champions) for 4 seconds he won't beat out Annie in damage (Annie has a 4 second CD on Disintegrate, totaling her damage to 1295). 4 x 240 = 960. However, Defile is usually on as well. 4 x 110 = 440. 960 + 440 = 1400. 1400 > 1295. Thus Karthus beats out Annie's magic damage over a 4 second duration. But if Karthus misses a LW... 1400-260 = 1140. 1140 < 1295. Karthus loses.
I don't give a rat's ass if I didn't factor in AP ratios or team fight situations. The point of that math lesson was to show you why you FUCKING HAVE TO BE ABLE TO AIM THIS SHIT TO BE USEFUL.
I will be going over this IN DEPTH later in this guide.
| Wall of Pain
| A Wall of Pain that slows people and reduces MR.
First off, I need to say that THIS SHIT IS BROKEN. Take a look at what this wall does at rank 5.
If you don't realize how broken that is you need to uninstall your game.
On top of that, it grants a 35 MR reduction as well. That sends 30 base MR carries into NEGATIVE MR. With your Sorc Boots and MPEN runes you will be hitting 30% more damage with your spells on anyone who has the wall debuff on them.
What does all those things make this skill?
I will be going over this IN DEPTH later in the guide.
| Deals aoe damage around Karthus and gives him back mana for every last hit.
Nice extra damage and mana regen gives Karthus some easier damage and more lane presence early game.
Turn on when an enemy walks in range of the skill. Turn off when they walk out. Don't forget to turn it off or it will drain your mana like Anivia's ult on crack.
I will NOT be going over this IN DEPTH later in the guide.
| This is a fun skill. I'm sure you all know what it does. It is probably why you are all reading this guide.
I will be going over this EXTREMELY IN DEPTH later in the guide.
Every time Karthus receives a major change (or even if nothing in particular happens- gotta keep this guide fresh as my toothpaste), I will post it here.
I'm pretty sure nothing happened. As usual.
Same as always- Karthus didn't get nerfed or buffed.
vSomething a month before v220.127.116.11
I'm not a fan of poring through patch notes to find the exact patch where Karthus was nerfed a few weeks ago, but here's the gist of it:
"The community QQ'ed and Riot, getting dumber every time they try to do something, listened."
Thus this totally balanced champion, who you all know as Karthus, got nerfed.
The AP ratio on his ult was lowered to 0.6 from 0.7.
My thoughts on this issue:
I don't give a fuck. The only thing I'm annoyed about is that Riot listened to QQ and touched this champion.
His passive was reduced from 8 to 7 seconds and it no longer reduces respawn time.
My thoughts on this issue:
I don't give a fuck. If you're playing smart, you would've walled as soon as you bit the dust and started pressing the R button. 1 more lay waste or 1 more second of Defile doesn't make a difference. It's the WALL and ULT that counts.
That's about it. Aside from the respect I've lost for Riot as a company, these nerfs don't actually affect Karthus terribly, if at all.
Masteries + Runes
9/0/21. Fairly standard masteries for a mage.
For runes, I roll with
9x Greater Mark of Insight
9x Greater Seal of Replenishment
9x Greater Glyph of Replenishment
3x Greater Quintessence of Fortitude
You can switch out the Mana Regen glyphs for something else, but I like having boatloads of solid mana regen during laning so that I can spam Wall of Pain.
Fortitude quints are a must have. A couple months ago when I still was terrible with Karthus, I went against a Karthus that could aim Lay Waste a hell of a lot better than me. But I killed him two times in mid. How? I had health quints and just rushed him with exhaust (this was before exhaust reduced magic damage; i won only because of the quints).
Taking flat health quints will put your hp on par with people who grab Doran's and ensure that you don't look like free food to anyone you are laning against. Every time I see an Ashe with 4 bars of hp I think "NOM NOM NOM."
MPEN marks and Mana Regen seals and glyphs (or just seals) are self-explanatory.
Take Exhaust and Flash.
Those are your only options. Don't take anything else. Ghost? No. Ignite? No. Clarity? Fuck you. You take Exhaust and Flash.
Because these two spells will ensure that you dominate and survive at the same time.
"LOLOLOLOL NOOB YOU ARE SO STUPID KARTHUS HAS 80% SLOW HAHAHAHA EXHAUST IS SO USELESS!"
Hey, bro, l2read. You don't take exhaust for the slow. You take it to wreck a single enemy target's damage output. Picture this: two equally farmed master yis with similar builds are duking it out in a bush. One of them has exhaust and uses it on the other. Who wins?
If you answered the yi that had the exhaust, congratulations, you understand how a summoner spell works.
If you answered the other yi, you need to take a trip down memory lane and ask yourself how you got to level 30 with only losses.
Now for a little harder scenario: what if the master yi with the exhaust had 50 less AD than the other master yi? He'd probably still win, as long as he still does a considerable amount of damage.
So, the use for exhaust is to pop it against anyone who has a significant advantage over you and crush them mercilessly. Karthus outputs so much magic dps mid to late game that it will seem like he is pulling off continuous Ryze combos, and on a crippled enemy dps (or nuker, but preferably dps for more damage reduction), Karthus will tear them apart before they can think about running away.
I also have an entire section on the best and funniest way to use Exhaust: Exhaust baiting.
Now on to flash. Why not ghost? Because you're not a tank. Tanks get away with ghost because they can survive the initial damage and considerable damage while they are being chased. Karthus doesn't survive while being chased. He rarely even survives the initial damage. He gains distance and gets out ASAP or he dies. End of Story.
First, start off with a Sapphire Crystal and 2 Health Potion.
From there, grab a Sorcerer's Shoes on your first port back.
If you don't have enough gold, grab a Ruby Crystal + Boots of Speed or just the boots.
After Sorcerer's boots, turn that Sapphire Crystal into a Catalyst the Protector and make that into a Rod of Ages ASAP.
After RoA, create a Rabadon's Deathcap for massive AP.
Afterwards, get a Void Staff because smart teammates will stack MR against you when you are doing well and have a lot of AP.
Then get an Archangel's Staff for even MORE AP.
As a final item, choose between Guardian Angel, Lich Bane, and Zhonya's Hourglass.
If the enemy team is mostly dps, you will laugh and kill them all with impunity assuming you can aim and place exhaust properly.
If the enemy nukers are focusing you, consider an Abyssal Scepter instead of Archangel's or even Voidstaff if no one is stacking MR.
I'm pretty sure every other Karthus guide that hasn't been written by a complete noob has a section on this item.
Archangel's, while synergizing well with Karthus's Lay Waste, will make you squishy as hell and leave you to be torn apart by everyone on the enemy team, IF YOU RUSH IT. Believe it or not, having less hp bars than your Teemo will cause enemy champion auto attacks and nukes to hit in your immediate vicinity (Unless you are playing with a Meeto, but then no one on your team will have that many HP bars).
The extra AP simply isn't comparable to the survivability gained with RoA.
And for people who rush AA and then grab a Rylai's... Karthus doesn't need a motherfucking item that slows. He has a FUCKING BROKEN 80% slow for 5 seconds (even though the slow tapers off at the end). On top of that, if you are following my guide, you also have FUCKING EXHAUST. There's absolutely no point in getting Rylai's when you can get a Deathcap and deal out considerably more damage.
As stated by numerous other Karthus players: You can tell the difference between a freeweek Karthus player and a dedicated Karthus player by whether they rush Archangel's or not.
Grab Lay Waste first to harass/farm before and after the first wave of minions arrive. Afterwards, grab wall since you don't need mana regen and don't have enough max mana to sustain defile at level 2. Take another point in LW at level 3, throw a point in Defile at lvl 4 for mana regen, and put a point in LW at level 5. Then take Requiem at level 6.
From then onwards:
Requiem > Lay Waste > Wall > Defile
Reason for this build.
"BUT DEFILE GIVES ME SO MUCH DMGZ MAN!"
L2P. Defile does shit base damage, about the same as your auto attack, and costs a fuckload of mana. Compared to a harder hitting cheaper Lay Waste and a 80% slow + 35 MR reduction wall, you are better off maxing LW and Wall first. Defile comes later when you are rocking 400+ AP and dishing out 200+ dps with your circle of death.
"I'M NOT A NOOB MAN! I GET LAY WASTE FIRST ALL THE TIME! BUT I DON'T GET WALL! NAW MAN I GET SO MANY KILL MID GAME WITH MY DEFILEZ DMGZ!!
As stated above, WoP gives a massive aoe slow and MR reduction, two things that are amazing during team fights, chasing, and escaping.
"WTF I DON'T CARE ABOUT MY TEAM I JUST WANT KILLZZZZ"
Pros / Cons
Massive consistent magic damage
Amazing aoe cc and mr reduction
Ridiculous ganking and chasing ability
Global ult that trolls the enemy team
Passive that requires you to die
Takes a long time to learn and master (200 some games to learn... still mastering at 400 some games)
How to Play Karthus
I am dividing this part of the guide into three sections:
All right, I will be honest. The passive isn't actually useless most of the time. It's really useful when you die. But you will be trying not to. Which means that in an ideal game, you won't use it at all.
General use of Passive:
After dying, throw down your wall, turn on defile, and start bombing away if you were ganked/teamfight has ended but no enemy champion is at ultable hp/you don't have your ult up. If the teamfight is still going on or an enemy champion is in ultable range, you requiem.
Your goal with Karthus's passive is to fuck up the enemy as much as possible after you die. Obviously you will be doing the most to the enemy team with Requiem and Wall of Pain so make sure that if you can, and will benefit, you should try to use those skills before you hit the floor like a sack of bones.
DON'T BE A SUICIDE BOMBER KARTHUS. Your goal is not to get 72 virgins in Muslim heaven. Your goal is to deal as much damage as possible to the enemy team. You can't do that by flashing in with Defile yelling "CAWCAWCAWCAWCAW."
You can do so much more by sticking to the side spamming LW and WoP and waiting for a good moment to go in with Defile/waiting for an enemy dps to come close enough to Defile. 15 seconds of living is greater than 8 seconds of being dead since you will still have 8 seconds of your passive no matter when you die.
Sometimes when your team is lacking damage (or needs more firepower) or sucking in general, you might have to get in there immediately and risk death. Generally this happens a few times during most games, which is why I don't advocate buying a Mejai's Soulstealer as part of my build. Karthus dies fast, and sometimes he has to.
So. This spell. Most of you aren't aware of how this spell ACTUALLY works. Lay Waste is a delayed aoe nuke with an EXTREMELY SMALL radius that does double damage if you hit one target. That's what you already know. What you don't know is that if you don't time it DOWN TO THE LAST MILLISECOND you won't hit the enemy. Also, if you have a slight bit of lag that gives you a slight delay before casting you will also NOT HIT THE ENEMY.
As explained at the top in the section with the badass lore, if you don't land all your Lay Wastes, you do shitty damage compared to other nukers. Which means there was no reason why you shouldn't have clicked the Ryze portrait during champion select.
The question is, can you learn how to hit with this spell 100% of the time, no matter whether the opponent is juking, ghosting, flashing, being slowed by differing ranks of WoP (40%,50%,60%,70% 80%), turning, running in circles... NO MATTER WHAT THE FUCK THEY ARE DOING? If you can't or you don't feel like it, I don't blame you. Just press alt+f4 and pick a new champion to learn.
Now you are probably thinking that I'm exaggerating when I say 100%. You're right. You don't need to hit with it 100% of the time when you are tossing it occasionally while laning or doing some pre-teamfight harassing. Then you only need about 50% accuracy. But when you are actually trying to kill someone, you HAVE TO to hit with this skill 100% of the time if you want the kill and want to live while getting it.
A few more things about this skill: use it to check bushes. Wall of Pain can check bushes too, but the cost and cooldown is too great for a bit more LOS.
Wall of Pain:
This is Karthus's every situation skill. Use this spell whenever you want to harass, kill, gank, set up teamfights, etc. As long as you have enough mana to follow up this skill with lots of Lay Wastes, this should never be off cool down when an enemy champion is in range of it.
This skill will disrupt teamfights and throw them in your favor (80% slow means the enemy team is practically standing still), piss off the enemy, set up massive damage harasses, and when used in conjunction with flash, pull off impossible kills.
Throw this down all the time, period. It has a short cooldown (the wall lasts for 7 seconds, and with golem, the spell technically has an 8 second cd). Learn to use this spell as your lifeblood.
DO NOT BE AFRAID TO USE THIS SPELL. If you aren't constantly using this spell (by constantly, I mean fucking SPAMMING it) while playing Karthus, you aren't playing Karthus.
Oh boy. Everyone's favorite and most hated skill. The skill that takes down that Kassadin after he ducks and weaves through the entire enemy team, being saved by a hair at the turret by Rammus's taunt. The skill that kills the Katarina but leaves the Ezreal at half hp while he's farming, spamming "OMG SO ANNOYING" in allchat.
250/400/550 base magic damage with a 0.7 AP ratio. 250 damage at the first rank? Total shit, amirite? Damn right, this ult does TOTAL SHIT DAMAGE at the first rank. What does this mean? When your Amumu is screaming "KARTHUS ULT ULT ULT ULT" after the 300 hp Master Yi gets away, you have to calmly explain to him that your ultimate only hits 200 damage and that Master Yi possesses Meditation as well, which gives him a considerable boost in MR. Then when the same Amumu is screaming for you to ult that 350 hp Miss Fortune when you hit level 11 and your ult is doing 400 base damage, you have to explain slightly louder that Miss Fortune picked up a Negatron Cloak and is blocking 40% of all incoming magic damage. Finally, when Amumu yells at you to ult that 50 hp Ashe, you must tell him to STFU and that there is a Morgana with maxed spellshield standing right next to her.
Wait slow down. You have to analyze your target before you ult?
"WHAT THE FUCK I HAVE TO ANALYZE MY TARGET? SCREW THIS I JUST WANTED SOME LULZ WITH TROLLULT!"
You bet your ass you have to analyze. Now get the fuck out of my guide.
For the rest of you, you have to assess a lot of things when ulting. For example, current health and magic resist.
Just to give you a heads up that every other Karthus guide gives, Requiem does 250 BASE magic damage. So with 30 base MR on the enemy champion, you're looking at roughly 200 magic damage. With boots and MPEN marks, you will hit very close to the full 250 damage (that's why boots before catalyst).
Assuming you don't have boots, you can't kill anyone above 200 health. It's even worse when they pick up Mercury's since you will be hitting even less.
Before ulting, click on the enemy champion and read their current health first, followed by their magic resist. You might need to scroll over the magic resist number to see the percent of magic damage blocked at first, but eventually you will get a feel for the numbers and know the amount of damage blocked with varying amounts of MR.
If the enemy champion is nearly out of your LOS and you only catch their hp, press "o" or tab to take a quick peek at their items. If you see that Malphite with a Force of Nature, it's time to take your finger off that R button (unless he is REALLY low).
Also, you must assess cooldowns. Many champions possess a shield, a heal, or a vlad pool. When your teammates get into a fight in a distant lane, you must watch the fight in its entirety or mostly to see if they use the skill that can block your ult. Most of the time they do. If you catch the teamfight at the end because of distractions while laning, you must make a decision whether to ult. Usually, people use their shield, heal, and escape move during a tough fight, so feel free to use it unless there is someone else protecting them after the fight (soraka, morgana, sona, etc.).
REMEMBER TO BE SAFE AND OUT OF ENEMY LOS WHEN YOU ULT! If you have to flash to get out of LOS, go ahead (only when in danger of being ganked or when a cc tank is blocking your chasing path). Especially in enemy territory. It's better to flash and get a kill than to get stopped by an Ashe arrow a moment before the red beam explodes. If your flash is on cd, and if you're ulting on a champion who has a low cd escape move, you can take a risk and ult in the open.
Most people think champions with moves that CAN block Karthus's ult ALWAYS block Karthus's ult. Not true. I haven't been blocked by a shield/escape move for few months. Except when Nocturne came out and I didn't know that he had a spell shield. So, don't get DIRECTLY* blocked by any move and be smart with your ult. Or stop playing Karthus because you are making me look bad.
Note: At rank 1 of Requiem, HEALTH POTIONS actually pose a significant threat, especially when ulting on a risky kill.
Special section for Soraka/Shen
These two nasty little champions have global ultimates that can save their teammates no matter where they are on the map. Make sure you see Soraka use "Wish" before casting Requiem and make sure Shen's shield dissipates (because it only lasts a few seconds) before pressing R.
*directly means getting blocked while the red beam is over the target's head (things like heals, escape moves, etc.) Indirectly getting blocked (Soraka heals your target before you start ulting) is okay because you don't lose your ult and can save it for later.
So you've bought your Mana Crystal and Health Potions and you are at your mid turret.
Oh yes, go mid. DO NOT GO INTO A FUCKING SIDE LANE UNLESS YOU HAVE PLAYED KARTHUS FOR HUNDREDS OF GAMES AND YOU DON'T NEED THE EXPERIENCE OF MIDDING WITH KARTHUS. HELL, EVEN IF YOU PLAYED KARTHUS FOR HUNDREDS OF GAMES, GO MID BECAUSE KARTHUS IS DAMN EFFECTIVE IN MID.
You can assist other lanes with your ult while standing in mid. Think about how useful it is to have a higher leveled Karthus on the team.
All right. To continue, go to the side bushes at mid (you know, the ones leading to the river) and throw some bombs in there to check for gankers. If it's all clear, head up to the enemy turret to see who you are up against. Sometimes, people will move to their turret and afk until the minions spawn. Kill them or punish them for doing so. If they aren't afk at their turret (99.99% of the time), harass them the best you can before minions spawn. If they just aren't at their turret yet, go back to your own and chill until the little guys come out of the nexus.
Once the minions are duking it out in mid, you can begin-
Use your autoattack as much as possible. It takes some getting used to, but don't worry about it. You'll learn. Throw in a couple Lay Wastes when you can't get to the minion in time to autoattack.
DO NOT FARM SOLELY WITH LAY WASTE. If you do, I will fucking bash your face in with Trundle's club.
An occasional Lay Waste to last hit is fine, actively spamming it on a group of minions is not.
Use Lay Waste to harass. In fact, spend most of your time harassing during the initial wave. You will probably push the other champion back to their turret by the third wave. While the enemy champion is trying to farm in between turret shots, harass them heavily to ensure that they receive as little gold as possible.
At level 2, pick up WoP and use it as soon as the other champion is in range. Follow up the wall with as many Lay Wastes as you can before the other champion reaches their turret. If they have Flash or Ghost, they will probably use it, depending on how much damage you dealt to them. You might actually kill them at level 2, but don't count on it.
At level 3, you will have 2 ranks of Lay Waste. Now you can actually kill the other champ. Once the other champ is at half health or so, WoP and legitimately try to kill them. You may or may not succeed, but they will be pretty damn hurt either way (assuming you have at least a little bit of skill in aiming your Q).
Try and keep them as low as possible. Most people aren't idiots and will go b when they are at 200 health and you are level 5 out of fear of your Requiem. If they don't, kill them when you level up. Most likely, you will just have a significant exp advantage over the enemy mid after they recall (or if you killed him/her earlier, a MASSIVE exp advantage).
Go for the kill all the time after level 3. At level 4, the puny amount of damage Defile does will still help you kill your opponent, so turn it on when you have the mana.
So you've hit level 6. Take that point in Requiem and look around the map. If nothing exciting is going on, focus on harassing your mid and farming. If it looks like a fight is starting/ending take the steps illustrated above in the Requiem section of Skills.
If you kill your mid and you still have a decent amount of mana and health, go out and gank a lane where the enemy is overextending. Do this also if your mid is missing and you see an opportunity. You can even try to gank when you've pushed the minions up to the enemy turret and your opponent is trying to destroy them. But DON'T do this when the enemy mid is close to your turret. I don't give a shit about your reason. That puny 2-3 hundred gold you got when you killed that Corki is not worth as much as a turret, which gives a good amount of gold to ALL of the enemy teammates.
The general way to gank as Karthus. Wait for the enemy teammates to stand still or move predictably. Throw down your wall and proceed to Defile and Lay Waste the hell out of them, using your Exhaust and Flash if needbe.
Ganking with Wall of Pain is much harder than using it at mid to harass.
Why? Because the wall is cast in a circle around you, and since most of the time you are coming in from the side when ganking, leaves no room for missing with your wall since the radius is a straight line. Here let me demonstrate.
See why this is fucking hard to do? Because if the enemy champions moves a hairsbreadth up or down, they dodge your wall. But if you pull it off, they can't run straight back to their turret since they will be stuck in your wall. Instead, they have to move up or down to escape, giving your allies a little more time to catch up.
More realistically, the enemy champion will be going diagonally from your wall to the bushes, avoiding touching the wall but heading left or right as much as possible. Still, forcing them to move up or down a little buys time for your Annie to come in and nuke their asses.
Why did I just explain something like that?
Because I'm showing you that aiming a wall while ganking from the side is FUCKING HARD TO DO but will ABSOLUTELY RAPE THE ENEMY .
One more thing about laning/ganking:
Take Golem as much as possible. When taking golem, put the wall on him for MR reduction and bomb away. Don't turn on Defile when jungling unless you have the mana and ap to make it worthwhile.
This is a very important skill to have, especially when laning, but important in all stages of the game.
Exhaust baiting: A melee or ranged dps attacks you, gaining the upper hand, when you pop exhaust and wreck their damage output and also leave them unable to escape as you bomb them to death.
When you should use it:
In direct 1v1 confrontations with an enemy dps. There is no baiting in teamfights or 2v1, 2v2, etc. Just 1v1. Generally, this occurs in mid, or when pushing a turret early game. This can also occur when an enemy Xin Zhao gets cocky and decides to randomly attack you in the jungle.
There are some rules to follow when Exhaust baiting.
Don't throw the Exhaust on someone with full health. They'll run. Assuming they aren't retarded.
Don't try this when your health is so low you can be one shotted. Know the enemy champions. Xin Zhao can throw down some considerable damage with his ult, so be careful when baiting him. Not to mention he can throw you up with Three Talon Strike.
Don't back out. Believe me, Exhaust reduces enough damage from a dps so that you can kill them easily with low hp. Unless you've been feeding like crazy.
Use Exhaust only after the opponent has flashed/ghosted. Also check for Heal and other Summoner Spells that may prevent you from scoring the kill (such as another Exhaust).
A few ways to use this tactic.
You've just killed mid and you are now pushing the mid turret. You see that top has called mia on Master Yi. You still possess 400 hp. What do you do? Keep pushing until you think they should be quite close. Then move back. If the mia champion is ganking you (highly likely), he will act now. Kill the champion. You are higher level and have exhaust. Easy peasy.
In general, I tend to throw down Exhaust when I hit half hp or lower. That way the enemy won't run away (usually), since he thinks he can still kill me.
Champs you may meet in mid
I've decided to put in a section for champions you might see in mid, and how to deal with them.
First, to assess your opponent, you must figure out whether they are a nuker or a dps. If they are a nuker, be extremely careful when harassing and going in for the kill. Most likely a nuker's offensive spell will hit a lot more than a Lay Waste, so trading hits is not a good idea. If you're against a dps, well, that's always fun.
Ashe: Ah, Ashe. Everyone's favorite champion to mid against. Including Karthus. The trick here is to dodge her volleys and return with Lay Waste + Wall. This ensures that Ashe stays at low hp and uses her escape move. Honestly, that's all there is to it. If you are in a tight spot with low hp, simply throw on the exhaust and laugh as she does nothing to you.
Caitlyn: Cait can either be really annoying and deadly or just really annoying. Caitlyn's harassing skill, that weird harpoon thing, does quite a bit of damage. But it has a slight delay to cast (hehe). As long as you can dodge this reasonably well, you should have no trouble harassing her. The trouble lies in killing the bitch. She has a net that springs her back a nice distance, far enough to keep you from getting to her in time, even if you walled. That means the only kills you will get on her are ones from direct confrontations. People aren't usually dumb. They know Karthus hits pretty hard. Chances are you won't be seeing her dead body lying on the floor a lot in mid. Try to zone Caitlyn out, keeping her away from your minions, and thus her exp and gold. That's pretty much all you can do most of the time against Cait.
Ezreal: Ez is a fun champion to mid against. Harassing him is similar to harassing Ashe; you dodge his attacks (mainly Mystic Shot and sometimes Essence Flux) and counter with Lay Waste + Wall. But I find that once Ez picks up Arcane Shift (his flash), throwing down the wall becomes a waste of mana. Instead, you just need to aim better without the wall and get him low as possible. When going in for the kill, place the wall where you think he will flash to, not on top of him. This is a little easier than it sounds, believe me. If he tries to confront you, toss an exhaust and explode him.
Miss Fortune: The real annoyance with Miss Fortune comes not from her active skills, but from her passive. Her passive gives her a considerable movement speed boost, which gives her a better chance of dodging your bombs, especially if she grabbed boots first. However, she loses this boost as soon as she gets attacked, so once you get her once, go in for the kill. That's generally the solution to dealing with her.
Pantheon: Free kill. Bomb, bomb, bomb, wall, exhaust, bomb more, win. Just make sure he doesn't out harass you, and you will be fine.
Annie: Annie is an interesting champion to mid against. She has a pretty strong stun, something that Karthus tends to dislike. Yet it takes her 5 spells to get it charged. Watch her, and if she has Pyromania up, be careful of a full combo. She can nuke down a lot of damage very quickly and harass pretty well with her Disintegrate. Fortunately, after she blows her skills, she's done for 4 seconds, giving you time to get in there and be a pedophile.
Anivia: Probably one of the worst champions for Karthus to mid against. Anivia, if starting out with a Doran's Ring, should not be messed with. If she's going with a Mana Crystal, you can do something. First of all, harass her very hard. Don't let Anivia get farmed. She's one deadly nuker, if played right. Second, dodge her Flash Frost. With most Anivia's, as long as you're not drunk or high, this is piss easy. Once in a blue moon; however, you meet one that double predicts you and places it right where you juke. Those ones are dangerous and should be regarded with the utmost caution. Third, back off when she hits 6. Even the noobiest Anivias can smack down a good 400 damage with her ult + icicle combo. If she gets off a Flash Frost while you are being slowed by the ult, you're pretty much done for. Make sure that if you're planning on killing her, that your plan comes to fruition by level 6. Another thing to bring to attention about Anivia: she has an egg. Which means you don't ult her until she uses her egg once. Believe me, it's not worth it. Unless she is REALLY overextending, which she shouldn't be, at such low hp. Occasionally you will find yourself at 50 hp and you're attacking her egg while her minions are firing on you. Go back to base. Minions are deadlier than you think. Remember, if she loses the egg once, she loses a lot of survivability.
Brand: Brand can harass a lot harder than you and can return your damage easily with a quick two spell combo. To deal with him, you want to be constantly alert and dodging his circle of fire and firebolt thing while harassing him with Lay Waste. One mistake, and the tables are turned, so make sure not to get hit. When going in for the kill, wait for him to try to harass you with circle of fire and firebolt thing then go in immediately with Lay Waste + Wall, since his spells will be on cooldown.
Malzahar: Oh boy. Depending on his starting item, Malz can either be a breeze or be the worst nightmare you've ever had. If Malz goes with Doran's, be prepared to have a tough laning phase. If he goes with something else, try to shut him down early, since he will have less health. With Malz, you want to harass him as hard as possible before level 6, hopefully killing him. Once he hits 6, be wary. His full silence, pool, nether grasp, ignite combo can melt you like butter, even if you are at full hp. I generally see Malz's with Flash and Ignite, so he could pull off this entire combo in an instant with flash. Watch. Out.
Ryze: Ryze is, or was, your typical drop your nuke and run champion. Now that he can cast Overload every two seconds, he can really mess you up. All he needs to do is point and click, and you will lose a good chunk of hp. If you try to retaliate, you will lose even more. Your best bet with Ryze is to harass him as best you can while staying out of range, and try to get him to overextend into your turret or go back to base. A nice gank is helpful here.
That's all I have for now. I might have left some common ones out, but I'm damn tired and I need to go to sleep. Feel free to request some champions in the comments section.
Don't begin teamfights with your ult.
"LOLOLOLOLOL NOOB HITTING THE CARRY TO HALF HP BEFORE TEAMFIGHTS IS HOW TO WIN HAHAHAHAH"
Actually... no. Because the enemy team can just run the fuck away. Not to mention their banshee's might be on.
You shouldn't be doing this unless your team is perfectly set up with your Amumu hiding in the bushes right behind the enemy team.
In fact, you shouldn't be doing this even if your team is perfectly set up. Why?
You will do more damage by being close to the teamfight and spamming Lay Waste with Defile on than by pressing your ult.
"LOLOLOL YOU MIGHT NOT HIT THE ENTIRE ENEMY TEAM IF YOU SPAM LAY WASTE AND DEFILE"
You have to be safe from the enemy team (out of sight, in most cases) to start casting your ult, or risk it being interrupted. Which means you have to spend 3 seconds ulting and probably another 4 seconds getting to the frontlines to start dealing some more damage with your spells.
If you are already at the front lines spamming your spells, you will be doing more damage to the enemy team in the long run. Not to mention the fact that if you die, you can just start ulting right there.
During early teamfights (levels 1-12 ish), stay back and spam Wall of Pain and Lay Waste. Try to focus your Lay Waste on the squishies (carries, nukers, underlevelled tanks with no items). Now the next part is important: stay alive and win.
Staying Alive and Winning: Three Different Types of Opponents
Don't keep bombing after you see that LeBlanc make a beeline for your head. If you can't survive a nuker's main combo + one more spell then don't fight them. Run away and ult if the team fight is still going on. That way you lure an enemy champ out of the fight but continue to deal damage to the other team.
If you see a nuker blow their full combo on a teammate or get hard cc'ed, go ahead and wreck them.
If it's just one, and no one is in a position to help him with hard cc, exhaust, defile, wall, and q. If wall isn't up, that's fine. Continue on. If exhaust isn't up... consider how fed the dps is.
If he's been rampaging around, slaying your top and bottom lane continuously, don't challenge him without exhaust.
If he's been passively farming, judge him by his champion. If he's Master Yiit's probably safe to kill him as Master Yi doesn't have any cc. If it's Pantheon or Xin Zhao, be careful, because those two have pretty devastating single target cc.
If he's been feeding or is underlevelled, go fight him.
Two or more dpses coming at you? How fed are you? Legendary? Okay, see how far you get. Not legendary? Run away.
Judge tanks by their hp bars early game. If they have around the same hp as their teammates, kill them when they challenge you during a teamfight. If they have a ton of hp, run away.
Why? Low hp tanks can't handle much damage and tanks do low damage as a whole. High hp tanks don't do much damage, but they can take enough hits to outlast you in the long run.
Don't focus the really beefed up tanks during a teamfight. Lots of times, your Wall of Pain will send a beefy tank's MR hurtling down into the nether regions during midgame, but don't count on it lategame.
Mid-Late Game Teamfights (levels 13-18)
By this point you should be maxed out in Wall of Pain and Lay Waste. Spam them during teamfights just like how you did early on. However, this time you can add Defile in the mix for a little more oomph. Walk into a cluster of enemies with Defile on AFTER they blow most of their cc. Pick the right time to go in. Once you've waded into the center of the teamfight, watch both you champion and your health. Once your health starts dropping, flash out. Pretty simple.
Oh, and what if you mistime and get focused down with cc? Pick where to die if you can. Preferably in the center of the teamfight. Teamfight still going on after you bit the dust? Ult. Go back to the Skills section for more details on death and Karthus's passive.
Karthus's main strength in teamfights comes from the massive 80% slow and MR reduction from his wall and his high magical dps, along with his ults from beyond the grave.
Note: If, for some reason, your team decides to get into a 4v5 teamfight without you (because you wouldn't leave your team in a dangerous situation to go farm, right?), just ult when and if the banshees are blown.
All right, this part of the guide is really simple.
Once you obtain enough wins to get good opponents, they will try to juke you.
So what do you do?
Learn to predict.
If Karthus is your first skillshot champion, have fun working hard for a few hundred games on perfecting prediction.
It took me around 200 games to get this down where I can predict where 99% of people will try to go. This was after I mained Morgana, Ezreal, and Anivia, all of which I did fairly well with. To be fair, this was level 28-30, so the opponents weren't very good jukers most of the time. I soon paid the price for not learning how to anticipate jukes after playing Karthus for a bit.
Not much to say here. Just another section on how difficult Karthus is to learn.
This guide is the culmination of my 400+ games with Karthus. I will be updating it whenever I feel that I need to, since I never manage to say everything the first time around. If you just skipped down to the build and skilling order, you will fail horribly and I will be laughing my head off as I bomb your ass straight to hell in mid.
I wrote this guide because I love Karthus and I want to change the public view on what kind of champion he is and how he should be played.
Let's be frank, people think Karthus is an easy mode one trick pony who has an annoying bomb.
I hope this guide has shown you that Karthus is an effective champion who requires high skill, has great utility, and outputs high damage magical dps.
One final note: I will continue to update this guide, even though I don't play Karthus as much anymore. (Face it, 400+ nearly straight games with one champ... exhausting). Leave a comment and a rating please- ask any questions you want and feel free to point out errors in my guide. I will try to respond promptly!