Welcome. I play Karthus  For some time, so I'd like to share my experiences. This buildwas created by my colleague Piotr (eng. Peter?), who shared it with me. Now im sharing with you.  Build has been tested and changed in many games.

You must ask yourself: Why do i even play Karthus?
If you come in the course of deliberation to conclusion that you play Karthus because he has good ulti, you can at the same time close this guide and change your character, becasue you will probably killshots all.
You must realize that the strength of Karthus is Defile and a well-thrown Bombs.


Why it names Kamikaze? Because it's a good name that describes the style of play that Karthus. To do a lot of AOE damage you need to know how skillfully kite , put Wall and throw bombs. You will be most useful player in the team, if you will acquire these skills.
Your massive damage of 250-270dmg /s can handle any opponent, especially those with the low amount of HP.

Damage 42.2 (+3.25 / per level)
Health 390 (+75 / per level)
Mana 270 (+61 / per level)
Move Speed 310
Armor 11 (+3.5 / per level)
Spell Block 30 (+0 / per level)
Health Regen 1.1 (+0.11 / per level)
Mana Regen 1.3 (+0.12 / per level)

Pros:
-the best farmer in the game
-quite  strong as  AP, especially with Wall and 
-extremaly usefull in teamfights


Cons:
-oponnents dont like you. They say that Karthus is noobish hero.
-you will be the first targeted player
-in lategame strong AD does'nt let you life (Master Yi, Tryndamere, Fiora)
-items dependent
-often banned on ranked games

+
I take these spells  only when I play in the middle.
Teleport: improves our quick returns. I usually buyTears of God and use that skill 1st time.
Flash: You can get away from the enemy if skillfully used the V-Maps. On the other hand, we can start our attack using a flash and doing combination:  AOE (E), Flash(F), Wall (W) and spam bombs (Q). Flash can also start your  teamfights. A team must start earlier, because enemies can extremly fast burst you. One second after start you leap into fighting area.

I advise in on lane games and . Teleport would'nt be necessary,  because someone always watch the turret.

Until u get 12 lvl (ACC) and be able to use Flash you should use .

Generally I dont use other spells.  I've seen a lot of people with  and . The first one is usefull to get firstblood. I think its probably best way to use that spell. I realy dont know why some people take Clarity. Until now I have not found an answer: Why? Karthus basically hasn't problem with mana especialy with gear presented below.

21/0/9

Offense:
-AP
-CD reduction
-Magic Penetration


Utility:
-reducet times spent death
-additional mana and regeneration
-Increases vision range of wards

9x 9 x 0.95MPR which gives 8.55 MPR. In game, Client reads the statistics equal 9. This is the best and only one choice. Many AP points  with resil reduction  gives great force impact.

9x Greater Seal of Vitality 9X 1.08 HP / lvl 9x which gives 19.44 hp at 18 lvl. Total  174.96 HP ≈ 175HP

You can also put here defense runes. For example, when you change the ad/tank mastery.

9x . 9 x 0.99AP = 8.91AP ≈ 9AP This rune has ofc pros and cons. We have to start 39 AP on 1st lvl (15 from Doran's Ring + 15 from quintes + 9 from glyphs). The disadvantage is that the value on level 7 is incomparable with the choice below.

9x 9 x 0.17AP/lvl = 27.54 AP ≈ 28AP on 18 lvl. The disadvantage is that Karthus until lvl 7  based only on Doran's ring.

3x 3X 4.95AP=14.85 AP ≈ 15 AP. As I wrote, it gives a sizeable boost in the early game.

Abilities

 - Karthus unleashes a delayed blast at a location, dealing damage to nearby enemies.
Creates a delayed blast at Karthus' cursor position. After 0.5 seconds, deals 40/60/80/100/120 (+0.3) magic damage to each nearby enemy, this deals double damage if it hits only a single unit.

One of the basic Karthus skills  that provides fast farm in earlygame. You can quickly kill the enemy. We try to throw bombs, into enemy or on his path. Sense in throwing bombs are gained during the time. When you're fighting vs melee character, it is best try to put Bombs on yourself. This ensures that the attack on the enemy.



 - Karthus creates a passable screen of leeching energy. Any enemy units that walk through the screen have their move speed, armor, and magic resistance reduced for a period.
Creates a 800/900/1000/1100/1200 distance wide wall at target location that lasts for 5 seconds. Enemies that pass through the wall have their Armor and Magic Resist reduced by 15/20/25/30/35 and Movement Speed reduced by 40/50/60/70/80% for 5 seconds (their Movement Speed gradually recovers over the duration).

This skill is used in two ways: defensive and offensive.  It allows you to slow down the enemy, to get a chance to escape from oppression. On the other hand it allows us to catch up enemy, reducing resistance, maeby kill someone. . It also has another very important use. Discover the area. Well located Wall will help Blitzcrank to grab and pull enemy; detect someone in the bushes.



- Karthus passively steals energy from his victims, gaining mana on each kill. Alternatively, Karthus can surround himself in the souls of his prey, dealing damage to nearby enemies, but quickly draining his own mana.
Toggle Off: When Karthus kills a unit, he restores 20/27/34/41/48 mana.

Toggle On: Drains 30/42/54/66/78 Mana to deal 30/50/70/90/110 (+0.25) magic damage to nearby enemies each second.

This is the strongest Karthus attack and our main skill.  550 range is almost the whole battlefield. Good danced will kill a lot of enemies. Trynda doing 1k dps in one person. AND SO WHAT? Karthus doing 270dmg/s on whole area (5 people). This ability is the most powerful AOE, which is not ulti. AND THIS PASSIV – OMG.


- After channeling for 3 seconds, Karthus deals damage to all enemy champions.
After channeling for 3 seconds, Karthus deals 250/400/550 (+0.6) magic damage to all enemy champions (regardless of distance).

Three simple rules!:

1. Use when you have a chance to kill your enemy, and you're standing too far to kill him on EWQ sequence.

2. When your teammates ask you for judgment, because they can't catch up the enemy.

3. Always after death. Why? Because it's interruptable by fear, stun, silence. Why when you die? Why not before the fight? Because opponents before fight have all survivalities: auras, bubbles, BV's, heals. They lose it during the fight. When you use ulti after death, they probably have nothing.

> > >

1. Invaluable item,  that adds Karthus large amount of HP (450-630), Mana (525-725), AP (60-80). This item provides us nice survi. It is relatively cheap and easy to make.

2. 45 AP, 400-1400 mana, 25 mana regeneration. We have possibility to stacking mana, which will be converted to AP. Only this will give us 30  bonus AP. It's very easy to stack 1000 mana, because we already have ToG. Bombs spamming only accelerates this process. Of course mana is needful to our AOE.

3. Only for 20 MPR points and some movement speed.

4. 140 AP + passive 30% increase AP. Total gain 155AP from Rabadon. Of course we get much more from a passive.

5. 100 AP 50 armor. However, the most important is active skill. It provides Karthus 2 sec. immune. It is very important because everyone wants to kill Karthus. So we have a chance to survive attack.

6.  80 AP, 500 HP. Our spell damage will slow the target by 15% for Bombs(cooldown time is below than 3 second) and 15% for AOE. I think only Master Yi or Zilean can get away in this situation. It also increase our HP. Most likely you will not have this item, probably because the game end before time runs out.

You can always build something else, such as  , which provides us MPR, or (50AP + 30AP for all friendly players in areaeffect + spellwamp).


Why I don't buy  or ? I dont buy MS because its AOE Karthus. His strength are teamfights. which begin in middle or late game. FFS dont use Ulti for KS. Follow three simple rules above.
You can buy MET if u have some money and game is nearly won. You really don't need more CD reduction.

Easy mode:

Basically she does'nt pose a big problems, because it is paper. However, remember that paper beats rock, so Ashe can destroy the enemy. Ashe is a good pusher. It's easy to stop her. We put a wall, turn on AOE and throw a scarybombs.

Its very easy fight. His AD debuff does'nt work on us, so you should'nt worry abut this. We must quickly bring him to the ground.

 It's very good champion, which may well strike us. Generally, it's very e***y destroyed him or force him once or twice to recall. When we get lvl 6, he has not recall.

Medium mode:

Anivia has a nice burst and igreat survi in early game. So you have to play very carefully. She's not killable without good gangs.


Annoying mode:

It's hard to plave a wall on her, due to her blinks. On the otherhand She can get close to you, do damage and get back.

I hate play against Stick. Perhaps it's the most annoying of all mid champions, which I know. Hitting him doesn't make much sense, because he can heal himself. Probably, He would move line, not our Karthus. You have to wait for  gangs.

 Nobody likes to play mirrors. Fight will be boring. Will win person who knows better own champion , have more luck, faster junglegang. Just like that.


Hardcore mode:
THE MAGNIFICENT FIVE
,  ,  , ,  . They kill on stuncombo, so watch their casts, moves and many more signs. You have to play very safely, dont push, dont play on their area and rules. Let them play on our part of lane and on our rules, patiently farming and waiting for gangs.

Farm: Farming is very important at the beginning. You do not have great skills. You just need a feel. Use autoatack or Bombs to kill minions. On 2nd or 3rd Bombs do a lot of damage. Very quickly stack ToG. Magicminions are weaker and also not generally hit by the others. Use AOE to kill large groups - a large amount of mana helps us to kill


Gang: Would be nice  to have jungler with stun (Sion, WW, Jax). We'll  do more dps from AOE. When jungler runs to your opponent u have to turn on your combination: AOE + Flash + Wall +BOOOOOOOOOMBS.  Try to be in battlearena. When opponent's running out and jungler cant overtake him, you have to cast your ulti in safeplaces (Oponnents cant interrupt it).  Cast Ulti only if it makes any sense -  the opponent hasn't HP, heal, bubbles, resist auras, Banshee's Veil.


Teamfights: I know it's wrong, but very often I initiate fights. Don't do this. Join the fight seconds after its start. Turn on your AOE, put Wall and spam BOOOOOOOMBS. Spam Bombs on strongest opponents – yes on strongest. The weaker ones will die from our teammates and from the AOE. AOE should cover the maximum  fightarea. 250-270dmg /s it's really hudge damage. If you die, you have to analyze very quickly situation: throw Bombs or banging Ulti?
You get most kills in teamfights. Usually 2-3, sometimes pentakills. That style have some cons. AOE can't be controlled, so you'll be fed  at the expense of  teammates. You'll need to carry the game, and you ofc aren't AD or Tank. With bad luck, you can also feed on yourself the enemy. Really bad luck, if it will be one person, especialy AD.

Thank you for reading this guide. I'm so sorry for my English mutilation. English isn't my native languages​​, so sorry for all the logical and grammatical fails.

I hope that this guide was very helpful, and this Kamikaze Karthus made ​​you a lot of fun, a lot of kills, huge amount of  assists.


I look forward to comments, not necessarily related to my English skills. THX 4 all votes.