Hi all!
Karthus was the first champion i've played in LoL.
It was love at first sight, a Lich, the most powerful and terrifying undead!
At the beginning i underestimate his passive, yes i was really good but not essential.
At the end......you will see.
I chose this title for a reason, have you ever played a mmorpg? the classic bombing mage?
Have fun with this guide and forgive my english (i'll try my best).

Pro:

-He is the real meaning of farm
-He doesn't fear death
-He can kill everyone, everywhere with ulti
-Massive damage in teamfights

Con:

-Require some enemy movement prediction (and aiming skill)
-Require great attention to all enemy on map (you teammates often forget your ulti)
-Focus isnt' a problem, just resign, you have to die.

Karthus passive, the core of your power.
Death is your ally, and your enemies doom.
If they don't focus you, they will die, if they focus yoy they will die anyway.
Remember the most important thing of this passive, you have 7 seconds, if u cast requiem first you'll waste 4 second of free spells.
You have to learn your times, press E, use W, spam Q, press R just in time = Max efficency


Karthus main skill
This skill allow you to really "lay waste" to all living things on map.
A must for farming groups of minions, or harass an enemy champion.
At low level try to don't spam so much, first because it deals low damage second you risk to finish your mana pool.
In mid try to alternate Lay Waste and normal attack to kill minions and regain mana with Defile.
When this skill reach lvl 2, you can start harass seriously the enemy.
You must use this skill where your enemy is going not where your enemy is.
If enemy chase you just [lay waste] in front of him, and iyou have alredy a [wall of pain]] can reverse a bad situation.
Often in front of the wall the enemy have the bad idea to run away slowed, in that moment you can take the initiative and hit them hard with Lay Waste.
Remeber another good use of this skill, keep away enemy from minions.


The name of this skill reflect perfectly the effect on enemies.
There are some uses of this skill in mid lane:

-block an enemy, most of the times he run away slowed and become a easy target for

Lay Waste or it makes the enemy to waste Flash
-slow the enemy ganker and give him a taste of Lay Waste.
-escape a big gank with the help of Flash
-reveal bushes or other zones and prevent ganks
-Make your team score and ace slowing running enemies

REMEMBER: Wall of Pain not only slow, but also lower all kind of defences -> easy kill for you and your teammates


The "bombing" skill of karthus!
Not so effective at low levels and at low AP.
Awesome passive, as i said before Defile can give you infinite mana if used in the correct way.
Never use defile to farm minion in lane at first or second level of the skill, you will lose too much mana and the damage output is too low, better use Lay Waste, good aoe and make you regain mana with defile passive, remeber multi kill with Q = more mana for you.
We will see the real bombing use in the next chapters.


The red ribbon of doom.
This is one of the most powerful skill in this game, but need much attention to be really effective.
First of all, you have to look all over the map.
As i said before often your teammates forget your ulti, so if you want a kill, pay attention to all lanes.
Second, care about your enemies and their defensive skills, i will not bother you with the names of all the champions that can save people from requiem, all healing/shields/invulnerability can be a pain for your ulti.
Banshee's Veil it isn't a big problem, you need only a good timing, instead give a look to enemy equip, if you see Zhonya's Hourglass make sure it is on cd before you waste requiem.

Let's see some summoner spells

Usually Karthus is a mid champion, and this is my personal summoner spells choice:


Flash is a must for the bombing undead, it allows you either to escape and.....to bomb.


"oh clarity noob, 2 clarity op, clarity gg" all the times! Come on we are mages, we use mana!
In mid you will end mana! And this underestimated spell allows you to save your life, or to make a little "surprise" to your enemy.

Other spells


You have requiem if you cannot kill them with this skill.....


You have the wall, you have the life after death, you don't need this.


You have wall for chase, you have life after death, why retreat?

forify



This spells aren't for you.


This spell have a sense for Karthus, you can experiment some strategy, you can spam Q in Death Defied then use Revive and give the final shot with Requiem.
Never sacrifice Flash for this spell.


If you are addicted to this skill you can use, same as before never sacrifice Flash for Teleport.

Let's analyze game phases

Usually mid is a must for karthus, but sometimes i played in lane with good result.
This trime you are mid.
In mid you have a classical ap caster and they have jungler.
First of all don't psma Q too much, don't waste your mana and don't push too much.
Farm all you can farm, last hit with auto attack or kill multiple minions with Q.
Don't use Defile for farm.
Harass your enemy in all way you know, don't allow him to give last hit to minions, use Wall of Pain to outdistance you enemy from minions. (all fear the wall of pain).
Make your enemy waste flash, slow him with wall fo pain and harass him qith Lay Waste, he usually escape with Flash.
When you are ready with mana, flash -> slow him spam Q, dodge his attacks using Flash, defeat him.
If he runs with 10 hp and you have Requiem you win anyway.

Particular situation with particualr champions


No particular trouble with her, just dodge hes Flash Frost at right time, she surely follow her skill to damage you with Frostbite, then just use Wall of Pain and kill her twice.


Before lvl 6 it's a joke, try to kill her as much you can, at lvl 6 begins problem.
She will have Flash for sure, she will try to flash and spamm all her skills on you.
Fortunatly her movement are predictable, just stay a bit defensive and when she flash near you be quick to flash next to her and then same combo as before.


Dodging skill is required for this flaming mage.
The trick is the same, when he pushes you with Sear and try a stun, dodge the attack with Flash and then Wall of Pain and Lay Waste.


This champion is annoying, never underestimate that poison.
They way is always the same, dodge all her poisons, harass with Lay Waste.
At the right time flash next her (closer you are better it is), when you are very close to her, she will have problem to cast her ulti correclty and you may have the chance to survive and own her.


Another annoying champion, with his rockets and turrets he can easy deny you from farming and harass you completly.
Play defensive, kill his turret when you can, Heimer weakness in early lane is mana, when you see him low on mana take initiative and try to kill him.


Again just dodge and she will have no hope against you.
Care about her shield, you may waste Requiem on it.


Harass him as much as you can, slow him and stay away!
If he takes you with the combo Overload Rune Prison Overload you will suffer a lot.
Remeber that Ryze ultimate give him spellvamp and can be really dangerous if you are among minions.


Easy enemy, when he use locus of power just run to him use Wall of Pain and spam Lay Waste.
Before lvl 6 after he uses Mage Chains and Arcanopulse (u have to dodge), he has a no skill time of about 4-5 seconds.
In this case Flash is usefull to dodge his ulti Arcane Barrage or Arcanopulse


Not really a big problem, just care about the "no mana" and "i heal every 5 seconds".
Don't be too much aggressive and remeber Lay Waste cd < Transfusion cd


Swain it's not so difficult to handle but he has the bad attitude to heal himself killing you.
It's a must to dodge his Nevermove if you are good to predict his movements it's not difficult to harass him with Lay Waste, as all enemies that snare you, give a look to gankers.


Another champion requiring dodging and movement skill.
Easy to harass with Lay Waste.
Care about her shield before use Requiem.


NOW BEGINS PROBLEMS


Play defensive, farm minions, and survive her damage output.
You can harass her at first 3-4 levels.
After level 5 she start to jump and use the annoying combo of sigil of silence Distortion and Ethereal Chains.
You foud yourslef silenced, snared, and when she reach lvl 6 just sigil of silence + Mimic deal tons of damage.
Try to avoid etheral chains and you have a chance to survive, buy hp and maybe mr if facing LeBlanc mid.


Similar to LeBlanc in attacking mechanics.
Can be easy handled to level 6, then he start Riftwalk, Null Sphere, Force Pulse........ No way you can sustain silence + slow for long time.
Ask for help as soon as you can.


This champion will make you suffer a lot.
She farm minions and you cannot do anything to avoid it, she has passive spellvamp and a antimagic shield that make hard to kill her with magic damage.
Dodge snares, avoid walking over Tormented Shadow (anyway she will hit you with this skill).
At lvl 6 play defensive, usually she use Tormented Shadowto kill minions then use Dark Binding, if she get you prepare to flash away as fast as you can, or Soul Shackles will put an end to your life.
Morgana is lethal when ganker apper, if she snare you are lost.


Another funny champion, he need only Malefic Visions and he can farm whatever he want, and if you are near you take dot too.
He can silence you and at lvl 6 he can suppress you.
Stay out of range, dodge silence and look at enemy Flash + Nether Grasp.


You need explaination about Veigar combo?
If he get you with Event Horizon in 2 second you will take in fae Dark Matter a Baleful Strike and a devastating Primordial Burst and you are an AP champ.

The real challenge


And so you are 1vs1 against another lich.
You have to prove your skill with Lay Waste and your dodging skill.
Remeber use well Wall of Pain because he have too.
Last thing, you will not the only with Requiem

Just after the Lane phase there is the Bombing time.
Ideal Equip for this time is:
doran ring, sorcerer shoes, catalyst the protector, Needlessly Large Rod

Leave mid lane, and start with a good gank if possibile.
Your level and equip usually allow you to deal great amount of punishment.
Theoretically u can easly own 2 enemies at bot or the solo top (maybe with help).
Just care about wards, wait the right momente and place a Wall of Pain to slow enemies.
The position of bushes allow you to place the wall pefectly.
After wall it's a spam of Lay Waste.

Teamfights mechanic with bombing Karthus.

First of all let your tank initiate, then use Wall of Pain not only to slow enemy, but to decrease their defence, with this you and your team can do more damage to all.
In teamfights no one will care to avoid the wall, and then you can start the bombing run.

First case, you don't have Zhonya's Hourglass: don't change at all, just Flash in the mid of fight with Defile active, spam Lay Waste

option 1: DIE, reactivate E, spam Q for 3 seconds then use Requiem

option 2: win teamfights before your death, use Requiem to clean up escaping enemies

DON'T USE REQUIEM NEVER IN TEAMFIGHTS WHILE YOU ARE ALIVE chance of being interrupted are to high.

Seconda case: you have Zhonya's Hourglass: don't change at all, flash in the mid of fight with [[defile active, when you see focus activate Zhonya, this will give you 2 seconds of free damage and more important at all your enemies will lose 2 seconds trying hitting you while your teammates attack.
Spam Q

option 1: DIE, reactivate E, spam Q for 3 seconds then use Requiem

option 2: win teamfights before your death, use Requiem to clean up escaping enemies

Of course there isn't only teamfights during the game.
Karthus can hanlde a lot of different situation, from 1 vs 1 to 1 vs N.
Pay attention to champions immune to slow, such as Master Yi, they can jump on you in a while.
In this case put on Defile e try to use Q in the enemy way, Zhonya's Hourglass can save your life too, with defile active, you have 2 second of free damage then you can use Flash e try to reverse the situation.
1 vs N tatic is simple, defend.
Use wall (long as all lane at lvl 5) to slow down and harass pushing enemies, right hits with Lay Waste can stop more fragile dps from push and you can earn time for your team arrival.
When your team is ready apply the Bombing Phase.

Some final notes about Karthus skills and tricks.

Wall of Pain can be used to save your teammates lives, at high level the amout of slow is really powerful, when enemies see the wall usually stop chasing, fearing a gank.
Wall of Pain can be used to reveal a big zone of map, u can scout blue from safe distance, u can scout baron and drake, u can reveal gankers in bushes and give 'em a taste of Lay Waste.
Lay Waste reveal enemies when hit them in bushes, use it to scout before go.
Learn to use Requiem when you are sure to kill, don't fully trust you teammates, look at enemies life, look at enemies mr, look at requiem damage and your magic penetration, then cast or not requiem.
Requiem can be used from safe distance when you teammates push hard, with high AP you will deal lot of damage to all enemies.
Requiem can be used to help your teammates in difficult, this skill can resolve a balanced situation or reverse a lost fight.

Conclusion:
I think this guide is one of many, but maybe it is somewaht original and challenging.
I found this style of play more interesting and satisfactory then a classic caster.
I hope you like this guide and the "bombing style" too.
See you!!!!!