Karthus: caster carry - by Disaster

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There are very few karthus guides out, so I thought I would share a build that gives me some good success.
I always get far more kills than deaths, a ton of assists, and am up there with the carries in damage done, if not surpassing them. The thing with karthus is that he is the only mage with high sustained damage, if you build him right. Note that I am, I think, midrange elo. I am a lv30 summoner and I typically play with/against summoners that are lv 30. I also tend to play Karthus a mix of balls out and conservative...I will never try to die and will always try to do my work and get the fk out. I have managed a 2.3-1-2 k/d/a ratio and have gone on 14-15 game winning streaks.
now, on to abilities:
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lay waste:
this is a very low mana ability with virtually no cooldown. like all of Karthus's abilities, it has a low-mid AP scaling. This DOES NOT mean you should not stack AP. The thing is, this is spammable to all hell. And, it does double damage to single targets (it has a small AOE range). The downside is that it has a 0.5 second delay. Anyway, LW is an excellent early farming tool, as well as one of the best harassing abilities in the game.

wall of pain:
This is a wall that slows anybody that passes through it for 5 seconds, as well as significantly lowering their armor and magic resistance. Positioning of this wall is a deal maker or breaker in team fights, small group fights, solo fights, and random fights at the turret. It is something that will enable karthus to get many, many, kills.

Does damage/second in an AoE effect around karthus, while draining mana/second
I think this is the bread and butter of karthus. While it gives karthus a huge KILL ME NAO sign, it also absolutely devastates your opponents. My karthus build focuses on making this spammable without burning through your mana. In combination with your other skills, this is what makes people say OH SHI* when they see a karthus running at them with a rammus initiating. Also, defile restores mana per enemy person/minion/creep slain when toggled off. Remember, turn off defile when you are done using it. or you will go oom, which is devastating.

the ability that generates the biggest lols. Basically channels for 4 seconds to do significant damage to everybody on the map. This ability has a pretty decent AP scaling of .6 or .7. I am able to ult for like 750 by end game if I am doing well, many times.

passive: defy death (or whatever its called)
basically, this is a controversial ability that allows you to cast for 8 seconds while dead. Many say this is amazing as it allows karthus to "get revenge", others say its a waste because they would rather have something that prevents karthus from dying in the first place. I like it, but I feel the same way as the latter.
Summoner Spells:
I tend to pick ghost and teleport. Ghost allows me to chase efficiently with defile and WoP, as well as escape from impending doom, again with WoP. Teleport gives me an early game advantage in xp and creep kills.

Harassing with Lay Waste:
There are two ways to harass with lay waste. One is to anticipate their path 0.5 to 1 second ahead of them, and lay waste there. Typically, the enemy will cry "oh shi* lay waste" and run into that path, and get hit by it anyway. Typically, try to aim it so that it hits just them, for double damage and double luls. If you miss, dont worry. While you are continuoutly lay wasting and last hitting minions, the enemy is prancing around like a moron not getting last hits and not hitting minions. 
The other way to lay waste, and this may be better if you are not experienced with karthus, is to wall of pain right on their face and lay waste on them in any direction they are moving. This typically guarantees the hit, but at the same time, puts your WoP on cooldown and burns through your mana quicker.

I found that there are two efficient methods, depending on whether or not your allies are there ready to assist you, and whether or not you need assistance in the first place. If this is like, a nasus with his ult up or something, WoP right behind him and rush in with lay waste and defile. Psychology dictates that the Nasus will try to avoid hitting the WoP and will either try to run around it, or will run the other way, into your enemies. Either way, your allies should be ready to pounce, as taking the long path will slow the enemy down even more than the WoP would.
The other efficient method would be to hit WoP on his face and rush in balls to the floor with defile and lay waste. If the enemy is at 50-60% and does not have sufficient healing/escaping abilities, you should have this in the bag.

Low Enemy HP:
They tend to run away, so requiem lol. But watch out here, because even through teemo may have 230 hp and lv1 requiem hits for 250, that teemo will crawl away with 3 hp. why? because of magic resist. Factor in magic resist and see if you can actually make the kill. Typically, unless they try to stack magic resist, anybody under 190hp should die for lv1 requiem. After that, its too variable to make any judgment calls.

Wall of Pain tips (besides ganking):
If you have an angry minotaur, a midget rat, and a little girl with a pedobear all running after you with every intent to defile your wall of pain, run. Of course, ghost helps here, but wall of pain before you do anything. Wall of pain right behind you, hit ghost, and run like the wind to the nearest vantage point, whether its heimers turrets, rammus, or your nexus.
If they are wailing away at your turret, you have two options. One is to WoP on their face and rush in with defile. That will keep them off your tower, but could also kill you. Another way is to Wall of Pain right behind them and start spamming lay waste on their faces from afar. If they run, they will either run around a giant wall, or be hit by it. which can turn into whatever (see other tips).
If you are facing enemies that are a bit out of your range, but have abilities to get around wall of pain (flash, tristana, yi, etc), then wall of pain on top of yourself. If they leap into you, they will get hit by it. Another strategy is to put it immediately behind you.

I find that its safe to solo golem around lf 7-8. Just defile his buddies, and then turn defile off and spam lay waste until he's dead. The golem buff pretty much makes defile spammable, especially if you have chalice.

****, I'm Dead:
Rule number 1: turn on defile.
After that, if there are people around, hit WoP on their face and start spamming LW. If its late game and respawn is like a minute, ult the last 4 seconds. The pressure generated from ult could easily turn the tide of many battles, or one, depending on how unified your team is.

UPDATE: Sniping:
This one is easy. Now lay waste, as its not a targeted ability, is an amazing sniping ability. Hiding in tall grass and sniping people with lay waste could absolutely devastate them. It may make sense that people will try to avoid putting themselves in a sniper zone, but it works better than most people would ever expect.

UPDATE: Ulting from base:
If you get someone very low and they run to safety, and you lack the mana to finish them off with your ult, port back to base. Often times they feel that they are free, and will try to regen health from lifesteal or atleast hesitate to return to base for atleast 5-6 seconds (maybe they want 100 more gold or an extra level first). They tend to forget one thing: you regen health/mana in your base. I have gotten tons of kills by getting them low, going oom, porting to base and ulting for the kill. It is tremendously effective.
skill order:

I go
1 Q
2 E
3 Q
4 W
5 Q
6 R
7 Q
8 E
and max Q, max E after that while taking ult when you can. lv 13-18 should be for maxing R and lvling W from 2-5.
my runes are eh because I spend my IP on heroes most of the time. Anyway, I tend to mix spell pen, cooldown reduction, and mana regen, emphasis on the first 2 (i like to have requiem up faster).
I typically start out with meki pendant, health pot, and mana pot. I then try to stay in my lane until I have enough cash for chalice and a sapphire crystal.
After that, I go in this order:
Boots 1
Rod of Ages
sorcerers shoes

after this, I survey my opponent and see how I'm doing. If i find myself in too many close calls, but seem to be doing fine on mana, I will get Rylai's scepter. The slow and added health will help my survivability significantly. If I find myself easily scoring kills and getting away, but have mana issues, I will go for archangel's staff.

Typically go for tear of the goddess first for archangels, or giant belt first for rylai's.

After I pick one, I see if my opponents are
a) stacking magic resistance
b) lots of casters (ESPECIALLY a skilled viegar)
c) just some schmucks

if a), I typically get void staff. if b), i typically go for abyssal scepter. if c), then I will probably grab archangel's.

After this, my final item is Zhonya's ring. typically the game is over by the time you start building this. but you should be well fed, so who knows 

Due to the feedback on my guide, I decided to try out a Mejai's Soulstealers build, and I must admit, I fell in love with it.

It is essentially the same as the above except instead of rushing Chalice, I rush Soulstealer. I would recommend that if you are new to the game and not good at last hitting, to NOT use this build, as the majority of your mana upkeep relies on last hitting (atleast early game, archangel's staff/golem fixes the problem). Also, I would highly recommend grabbing the golem buff every time it is up (my personal play style involves liberal use of defile, though I continuously switch it on/off). 

Start with Doran's ring and a health pot. Doran's ring gives me some wimpy mana regen that will do for now, as well as an early AP boost and some health. I will always request the solo mid, as it is essential to get ult up early to rack up some early kills for mejai's. I then get amplifying tome and build it into mejai's. Now, I tend to b alot since I am an overly cautious player. Many times, I dont have to b at all until I can outright afford mejai's. Sometimes, if I get an early kill and a push seems to be out of the question, I will b in that time and pick up amplifying tome and a ton of health pots, and next time get mejai's.

After that, follow the guide I posted before, where I start building boots/rod of ages, etc. (essentially ignore the chalice).

Now, early game, Harass the hell out of your opponent, but do not chase or initiate. You are still a weak, puny, squishy, girlyman caster. Focus on last hitting with your autoattack and lay waste. If your opponent gets really low, drop a wall on him and defile your heart away while hitting lay waste. You will probably go oom or very close to it, but its ok because he will be dead. 

Once you hit level 6, keep laning like usual. Except keep scouring your map for low hp runners to requiem. Map awareness is key here, you MUST know whats going on everywhere at once in order to be effective at clutch requiems. I solo queue 95% of the time, so relying on your teammates to call ult is an exercise in futility. Now, one thing you must realize is that you DONT HAVE TO SCORE KILLS WITH REQUIEM. I'll explain. Your friends master yi and alistar are laning top and you are laning bottom. They are at 75% health and their opponents are at 50-60% health. Hitting requiem will get both of them low, leaving alistar and yi to easily either finish them off, or them to fall back, giving ali/yi the laning advantage. This could lead to a fed yi (which is just as awesome as a fed karthus), a tower kill, or both. Not to mention that you still get assists and subsequently assist gold.

By lv 7-8, you should have 1-3 kills under your belt. At this pont, solo golem so you could get enough regen to spam defile that much more, and your requiem will be up faster. Then wander around ganking, scoring kills. If your opponent is a teemo or something, you could just run into a lane, gank him at 80% by WoP and defile/LW, and kill him. rinse, repeat. Once you get it down you will realize that its redic easy to gank as karthus and you should be dominating and ggggodlike.

My general strategy mid-late game is to keep golem buff up and run into team fights right after the tank/initiator, hitting defile as you run in and immediately WoP either right behind them or on their face (preferably on their face). then proceed to spam lay waste as much as you can. Once somebody is killed, hit ghost and run out, lay wasting. If its a losing teamfight and everybody is running, hit WoP right behind you as you are running. This really saves your teammates' lives.
By following this guide, hopefully you will become a force to be reckoned with.

Oh yeah, and people that it really sucks to lane against are: all stealth. Maybe it's a personal thing, but I just hate them all so much. If you are up against one that isnt awful, go ahead and get a ward (not an elixir, elixirs are too expensive for early game)

Also please do not flame, though I am open to criticisms.