So you have chosen to play Karthus. This is a good call as he has one of the most overpowered spells in the game. This coupled with one of the most ridiculous passives in the game makes him for extremely scary opposition. This build is designed to do what Karthus does best, BRING THE PAIN!!!!

(Q) Lay waste - 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.
Cost
20/26/32/38/44 Mana

This is Karthus' bread and butter it takes practice to use. It has 2 uses one hit the enemy champs hard if you catch them alone. even if you don't the cooldown is a second so you can try to hit them again with it as soon it comes back up. Early game though only hit as you can. You have to learn to anticipate their movements and hit them as often as you can. As a general rule aim behind them slightly so even if they step back it will still hit them. If they get clever switch things up and always keep them guessing when and where your going to hit

The other is last hitting minions when you score a last hit on a minion with this spell it activates defiles passive and gives you some mana for each minions killed so if you kill 3 minions with one use you should get back far more mana than what you spent to cast it.

(W) Wall of Pain - 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. Enemy champions that pass through the wall have their armor and magic resistance reduced by 15/20/25/30/35 and movement speed reduced by 40/50/60/70/80% (their move speed slowly returns to normal over the duration). The wall lasts 7 seconds.
Cost
100/100/100/100/100 Mana

Many people wonder why this is call the Wall of Pain it doesn't do any damage. That's because they underestimate this wall severely and this spells their doom. This wall brings then enemy to a crawl. It can be used as offensively and defensively. Offensivly it will allow you to jump on the enemy and lay waste them into oblivion the Magic resistance reduction on this will (especially early game) will reduce their MR to near 0 making your lay wastes do maximum damage.

(E) Defile - 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.
Cost 30/42/54/66/78 Mana Per Second

This is spells passive is SO useful and why last hitting is so important. Many people absolutely fail to understand how I can mid up to level 6 without having to go back for more mana and it's because of this. The active is less useful early game EXCEPT when you die as soon as you die hit E DON'T EVEN THINK ABOUT IT just do it Ive gotten quadra kills in team fights because of this but it eats mana like a like a fat kid in a donut shop.

(R) Requiem - After channeling for 3 seconds, Karthus deals damage to all enemy champions.
After channeling for 3 seconds, Karthus deals 250/400/550 (+0.7) magic damage to all enemy champions (regardless of distance).
Cost
150/175/200 Mana

This ultimate is total ******** and I mean total. You can run but you can't hide. There are some downsides however
1. If you are stunned you will stop casting and lose the ult and probably a kill. SO DON'T CAST AROUND ASHE (or anyone else with a stun) OR SHE WILL PUT AN ARROW DOWN YOUR THROAT!!!
2. You are vulnerable while you cast this spell and if you die before you finish you will lose it. So for the love of all things holy do not cast this in the middle of the field while the enemy champ is just sitting there waiting to put a damper on your day. IF you witness this happen on the other team sit back relax and enjoy a pint cause their Karthus is TOTAL FAIL. Alternativly if your on a team with a Karthus that does it just surrender out of sheer horror of their stupidity or pray the other team is fail cause this dumbass is just gonna be dead weight.
3. It cooldown is approximately an hour and a half. I exaggerate but seriously it takes forever and a day to come back up so don't just cast it willy nilly hoping for a kill cause in all probability it won't happen you gotta wait and have some lane awareness.

Death Defied - Upon dying, Karthus enters a spirit form allowing him to continue casting spells while dead for 8 seconds. Karthus spends 20% less time dead.

This passive is total ******** and coupled with his ult equates to absolute ******** that no one can do anything about. If you jump someone and it goes totally wrong or you know your going to lose but can't escape, Then BUM RUSH THEM it will totally offset the enemy seeing a kamakazi lich racing towards them at full speed lay wasting everywhere without a concern for his own safety. AS SOON AS YOU DIE hit E i cannot stress this enough and cast lay waste until you cant any more by this point they should be close to dead so ult an make sure they remeber why Karthus is so damn scary cause even when your dead your still casting and they cannot stop you.

I have a very strange way of playing Karthus it will defy the logic of many people who play him and swear by certain items *cough* majies *cough* that i understand why they like but i prefer this due to the sheer insanity of the damage output. As end game you will have 1000 AP.

Pros of Karthus
- He hurts, they have nerfed him and he still hits like a truck if played right. His q is a constant nuke (albeit less so now) and his ult is total and absolute ********. When you finish this build you will be doing 1300 (before taking into account Magic resistance) to all 5 champions on the enemy team

- His wall will put a damper on the enemies day it slows and drops both there mr and armor so the carries will love you.

- He is one of the best mids in the game.

- While i already mention his ult it also needs its separate explanation if you have a few good teammates who can call out a person who is almost dead and it will kill them and they can't escape from it. unlike other champs with ridiculous range ults like ashe, lux, or even kog'maw this ult cant miss, be dodged or stealthed out of. That twitch sneak away with low health? Won't matter cause he is going to die anyway.

- His passive is total ********. When he dies he can still cast for 8 seconds AND his skills cost no mana while like this. This means if you can kill that champ that killed you with less than 100 health left and finish the bastard off for killing and bring him with you. You also gain 20% less time spent dead which is always nice because if your mid and you and the enemy die you get back faster which means a slight advantage.

- He is an excellent pusher and can kill minions with his lay waste and his defile.

Cons
- Extremely squishy
- Very slow
- Extremely difficult to master and play i have seen so many karths that are so bad i want to throw my self against the nearest wall and knock myself unconscious.
- If used poorly he will eat through mana quickly but not necessarily if you play him well
- By himself he does almost no damage to the turret. He needs to either lead minions there or go with a buddy but by himself it takes minutes to kill the turret.

Masteries
Many people who have played this game for awhile are wondering WTF is with the 9 points in defense thats a total waste. Do me a favor scroll back up and read what strength of spirit does (its the one with the tree picture) I'll wait.
This still won't dawn on you probably. One of the major problems with being squishy and midding is that you are constantly running out of health. This boost allows you to stay mid longer and regen health much faster than your average caster allowing you to stay longer and deal more damage. It seems strange i know but it works. The armor and MR don't help much but every little bit helps. Alternatively you can take the 2 points out of armor and throw them in awareness or whereever
The rest are fairly standard caster masteries focusing on mana or cooldown with a dump in the time spent dead to give you almost 25% less time dead.

Runes
Marks: Good boost to hit them harder early on with you lay wast and also give a nice damage increase when bringing down the ult on some poor fools head

Seals: Gives a small boost of AP early game to help put the hurt on early game and counter his slow start.

Glyphs: Cool down reduction is your friend. It makes your q spammier but more importantly makes your ult come up faster so you can hit them again and again with it.

Quints: Same as the glyphs just a bigger boost together they will knock off about 10% of time spent waiting for your ult

Summoner spells:
Good Ones:
Ignite - It does true damage and a lot of it to give you a kill early game especially if your harassing them well. Plus while its down it gives you 10 ap boost which you will probably be casting if this doesn't kill them.

Exhaust - It slows and blinds which by themselves make this an amazing spell especially against ranged DPS like Twitch, Ashe, and Miss Fortune as it slows them so you can run and they can hit you. More importantly however if you take the mastery buff it reduces their armor and MR this coupled with the Wall of pain really lets you dish out the hurt against whoever you are laning against.

Other options:
Ghost - Good alternative to exhaust if you want better escapability and allows you to chase with your defile if they are running away.

Flash - Also a good alternative if you feel a little to vulnerable could also be used offensively to get in with defile in a team fight but that usually isn't a good idea.

Decent choices:

Teleport - All around good summoner spell good for getting back to mid if you really needed to buy or heal and get back quickly also good to defend a turret from a push or creep wave by teleporting defiling the creeps/ enemy champs into oblivion and jumping back to bast

Clairvoyance - also a decent choice if you want to see exacyly how much health an enemy has before you ult his is a great way to do it. Also gives you team a way to check baron and dragon with out walking into a gank.

Meh choices:

Cleasne - While i have never seen it done it could conceivably be used with his wall to escape as the enemy will think you trapped and suddenly they have been slowed and your already making your escape to the turret. So if your worried about stuns or slows you can be the first to try it

Bad choices:

Clarify - Just No..... If you need clarify then you are doing something wrong because karth has plenty of mana regen and this is a waste of a summoner spell Cleanse is better than this waste.

Revive - Hell No..... On Karthus really he already revives faster than most so WTF you need this for.

Heal - *facepalm* why are you even looking at this one probably need to play a tank because if you need to use this on Karthus your already screwed 8 ways to sunday don't bother

Item: The rush of boots is for 2 reasons. The early speed boost is nice as it gives you more maneuverability. It lets you move in faster get your ult off before they can react to the lay wastes delay.
The second reason is that the sorcerer shoe's give you a better damage boost early game then more AP will as the main problem early on is pushing through their MR to do maximum damage. They have also nerfed Karthus' lay waste hard so the AP ratio is much worse so it's better to do the true damage by breaking through their MR then it is to stack smaller AP items early on.

Now many are wondering why I am stacking Archangel's staffs. Most look and go mana gained by casting spells is a unique is limited to one staff and their is only a small amount of AP gained by this items base of 45 AP. WRONG!!!! If you read carefully it has a stackable passive that reads 3% of max mana is converted into AP. This is were the bulk of the AP comes from in this build when it is coupled with Zhonyas ring equating out to about 1000 AP every game that you finish this build. That equates out to about 1300 damage dealt by your ult end game. This is why stacking Archangel's staffs will give you so much AP.

Play Style
Early game 1-6:
At level one play conservatively once you reach level 2 start last hitting minions like your life depends on it cause it does. You should focus you lay waste on the enemy champion trying to catch them by themselves. If they start getting smart and stay next to the minions HIT THEM ANYWAY make them pay for this insolence and it will scare the hell out of them.
Once you reach level 4 this is when you maybe can score your first kill mid depending on how you get you are harassing of them is going. You have to look at you health and look at theirs judge how many of each kind of minions there are on the field. Their health should be much lower then yours at least below half at about a third preferably. Its takes some practice to judge whether or not you can survive the encounter even if you cant you can still probably finish them off. If you think the time is right its time for the kill combo.
Kill combo: Wall of Pain right on top of them or just behind them, exhaust them so they are slowed and (their MR has dropped to 0 or near 0 at this point coupled with the magic penetration rune and sorcerer boot's that should be close to fished after your first kill) and start to lay waste them to oblivion trying to deal the double damage on them (usually when you first drop the wall they immediatly try to back up so this has to happen very fast so the MR debuff is still on them). After the assault they still are somehow standing ignite them and they should be dead.

Here is the simplified version
-Wall
-Exhaust
-Lay Wast * 1,000,000
-ignite

Note: I did not include the use of defile on purpose at this level it drains your mana too fast make it worth it. You are better off using lay waste to as it should be doing far more damage per second anyway especially if you are singling them our for the double damage buff. Once you reach tear of the goddess you shouldn't have mana problems but use this sparingly

Ground rules when using Karthus' Ult: This takes far more skill than people give it credit for. When you cast you are vulnerable and it can be countered with stun/taunt. I will reiterate as i stated earlier to drive this point home: NEVER cast your ult within sight range of someone with a stun/taunt (or in the case of a champion like ashe within obvious area where she can blindly fire her GIANT STUN ARROW OF DOOM and put a damper on your day).

Further more if you die while casting you lose the spell and have to wait for the cooldown. SO DON'T CAST WHEN HER HEALTH IS LOW AND EZ OR ASHE (as examples) KNOW ABOUT WHERE YOU ARE. Their ults will kill you and you miss out on a kill and have to wait for the cooldown so you cant cast it after you have died and you gave them kill.

Actually using Karthus' ult:
This skill takes timing to use if you want to use it for killing especially early on. It says it does 250 damage to every enemy champ this is a lie you have to take MR into accout. I will use an example lets say you have bought Sorcerer's Boots so you essentially drop their MR by 20 points along with what you get with your magic penetration from your runes of 8.5. this will drop most squishy champions magic resistance to near 0. If a champion has 35 MR this brings actual damage dealt to 249.86 (if my math is right corrct me I am wrong) however most champions don't have 35 MR they have more then this.

As a general rule of thumb make sure you have them at 200 health (150 ideally) if you don't want to do the long calculations.

This takes lane awareness you can use the minimap to check on the other lanes but sometimes you think you are scrolling over a champion and may in fact be looking at the health of the minion. So i find it more effective to physically look at a lane fast while behind the minions or near the turret see if of the enemy champions are at low as soon as you are 6 if they are low enough and they don't have any of the problems mentioned below then BRING THE PAIN down on them.

PROBLEMS you may encounter:
You have to account for heals, Soraka is essentially the anti-Karthus with her ult, Nausus also gains health when he casts his ultimate. Also people with shields like Shen's feint as well as his ultimate. Many a potential kills have been foiled because people failed to take into account for heal spells or other means of protection from Karthus' ultimate.

Zhonya's ring: This gets it's own separate section because it is so infuriating and so common espcially on casters and other squishies like Akali. If you know someone has Zhonya's ring they are annoying as ****!!!!! They will drop that without fail as soon as they see you ult coming down. You only way to combat against this is to catch up to them before it wears off and put your wall up to prevent their escape or force them to use it close to your team mates so they can cut them off.

Mid - Late Game (11+):
At this point you should be finishing Zhonya's or almost finishing your second Archangle.
Mid to late game is basically team fights and ganking (Karthus' version of a gank is wait for them to escape and drop his ult he doesn't even need to move its great). When a team fight is about to begin ready your wall soon as the initiators initiate drop you wall trying to focus the squishies the most as the tanks should have so much mr and armor it won't make much of a difference as it will with the squishies. After this you have 2 options either stay in back and lay waste like a ***********er or be bait.
Being bait is a highly underrated approach as it will probably get you killed but can mean the difference in a team fight. When you die you are not out of the picture. Get right in the middle of them with your defile and make them focus you and when you die keep casting with wild abandon and then drop your ult with the last 3 seconds to spare because once you hit 6 seconds the ult becomes unavailable.