Soo, this may seem like a wonky guide, but this is the way I've played karthus for quite a while now, and thus far, my ratio of wins::losses is about 20::1, maybe more. I have yet to lose a ranked game with Karthus, though I'll admit, I haven't played that many...yet.
If you're still reading this, and you want a very unique and aggressive way to play one of the strongest (imho) casters in LoL, then this is the guide for you!

EDITS:

3/14/11 - Decided to finally take some time and make the guide pretty looking. =D Yay for pretty pictures!

4/27/11 - Have been testing with Void Staff much more, and will finally incorporate it into the build. More often than not it's more useful then the archangel's...though we're still stacking them for the mana. Also, changed up the beginning rune page to compensate for void so we can hit harder early game.

& - These are to utilize how hard will hit early game. I've been considering as well, but I haven't tried it yet. More to come =P

& - Pretty obvious. The amount of AP that Karthus will have mid/late game, we need at least a little magic penetration so most of that damage will get through. For most games, I think this will be enough. Your object isn't to take on the tanks, it's to take out carries as soon as possible, so a minimal amount serves just fine. We can edit the items we take later if they try to compensate.

The masteries are, for the most part, pretty typical for a caster dps. Nine in the offensive tree for the ability power, cooldowns, and magic pen. If you're not a fan of flash, and prefer even more offense, you can take cripple instead of the crit chance. I prefer flash, but I'll get into that in summoner spells.
The utility tree, in my opinion, can be messed with, that's the way I prefer it, that's all.

For the summoner spells, I prefer flash and clarity. Flash is amazing on almost all champs (I can't think of a champ which it's not awesome (and yes, I think kassadin is a perfectly viable choice for flash, too =P)), and provides an escape option AND an offensive surprise for your opponents, but I'll get to that later.
Some people will say that a good karthus doesn't need clarity because of the passive on Defile. Honestly, I think that's a bunch of ****. You're going to be harassing that opponent with Lay Waste as often as possible, and you WILL run out of mana. If you're anything like me, you'll do it shortly after you hit level two, lol. The (relatively short) cooldown of it and the range of Lay Waste give you the opportunity to stay in lane almost forever.
Some other options I could definitely see with Karthus:
- I've taken exhaust with karthus before, though I'm not a big fan of it. You can utilize the mastery for it, and make your spells hit even harder, and with your wall, your opponents will be at a crawl to try and get away.
Heal - Yes, heal. Before I truly utilized Flash, I took heal-clarity on Karthus for almost infinite laning power. Kinda lost interest in using Heal, but I won't discount it.
- I've never used teleport on him before, but he's called the rich champ for a reason; getting to those minion waves and clearing them fast will not only protect your towers, but give you so much more mobility. Not better than clarity or flash imho, but if you want mobility, I'm sure this would work for your play style.

Although this is (in my honest opinion) an advanced Karthus user guide, I'll go over a quick synopsis of his abilities anyways.

: Upon dying, Karthus enters a spirit form allowing him to continue casting spells while dead for 8 seconds.
This is why Karthus is so good. You'll be utilizing this in team fights, making sure you're the one focused first so all of your damage abilities can't be interrupted. The opposing team WILL make the mistake of killing you first.

: Creates a delayed blast at Karthus' cursor position. After 0.5 seconds, deals 40/60/80/100/120 (+30% of ability power) magic damage to each nearby enemy, this deals double damage if it hits only a single unit.
The cream of the crop. This is your main harassing ability in lanes. The range on this ability is 850, and many opponents will underestimate this. Take advantage and try to pop the ability behind them, unless they're too aggressive, and you've taken too much damage. Then you can use this to deter them from getting close. The kicker of this spell is the fact that it DEALS DOUBLE DAMAGE TO A SINGLE TARGET. Use this to your advantage.

: Creates a 800/900/1000/1100/1200 distance wide wall through target point. Reduces enemy armor and magic resistance by 15/20/25/30/35 and reduces movement speed by up to 40/50/60/70/80%; debuff lasts 5 seconds. The wall lasts 7 seconds.
Yes, it's a very good ability, and it's good to get early, but taking it early will digress from the play style that this guide is going for. It is very integral to max out and before grabbing this, please just believe me on this. Late game, you'll be using this to make sure the opposing team doesn't escape after the team fight in your defile and/or to lower their magic resistance.

: Passive: When Karthus kills a unit, he restores 20/27/34/41/48 mana.
Active: Drains 30/42/54/66/78 Mana to deal 30/50/70/90/110 (+25% of ability power) damage to nearby enemies each second.
Another amazing ability. It's passive will help you, but until it has higher ranks, it won't help that much. And it's utility early game isn't that great, but does have it's uses. If you're actually managing to hit your opponent with lay waste and they aren't backing off, you can use to get close and turn on to really hurt them and use to finish them off, even if they're behind their tower. Later game, you'll be turning this on almost all the time. See a huge minion wave begging for attention? Turn on, move around, turn off. You now have about 300+ more gold then you started with! Enemy trying to 3v2 you? Turn on, watch them go home crying. Die in a team fight? Turn on and put up preventing people from moving out of it. Watch them cry as either you or your team finishes them off.

: After channeling for 3 seconds, Karthus deals 250/400/550 (+70% of ability power) magic damage to all enemy champions (regardless of distance).
People will call you a noob for this ultimate. And, people will tell you that this is why Karthus is the #1 kill stealer in LoL. I'm here to tell you it's much more than that. Yes, you will be asking your team to essentially hand you kills with this, because a fed karthus can carry a team (I have done this more often than not), but it has much more utility than that. In a top or bottom gank by an enemy jungler, or if the enemy is tower diving top or bottom, you can use to force them to back off, giving your harassed teammates some time to recuperate. But the main time that you will be using this is when you're dead. Because of , you'll have 8 seconds of un-interrupted casting after your inevitable death in a team fight (since you've initiated that team fight, hence the name of the guide =D). Five of those seconds will be spamming and using to prevent escapes. The other three will be casting to either kill multiple opponents or weakening them enough for your team to finish. I like using the first 3 seconds of death to cast because your opponents will still be in defile most likely, so this will maximize damage output, but I don't think it would really matter when, as long as it goes off.

Okay, I'll be honest, the skill sequence I've given above, I'm not positive it's the right one, but it's close. The best way to think about skills for Karthus is a priority issue. In other words, ask yourself these questions when you level:
1.) Can you level ? If yes, do so, if not, proceed to question two.
2.) Can you level ? If yes, do so, if not, proceed to question three.
3.) Can you level ? If yes, do so, if not, level .
That's what I follow, and though it'll get you your wall at level 13 (pretty sure it's 13), your other abilities will be hitting so hard, your opponents will be waking up next week.

To start off with, grab a Mana Sapphire and a and . I've been toying with the prospect of taking two health pots instead one of each, but I've been doing so well lately, I figure, why try to fix something that's not broken? LOL
From there, I usually don't leave the lane until I can make , but if you have to, try to stay until you can at least make Tears of the Goddess and a . Tears is integral early game, since you'll be spamming . From there, boots of speed should do well. Oh, something I forgot to put in the item order: . Amazing item. I usually grab one after I make the first , after Rabadon's Death Cap, and pretty much every time I go back (if I can afford it). The extra CDR and ability power is integral as well. Rabadon's Death Cap is a must, and from there, I stack s. And yes, I've done the math, stacking archangel's is actually more efficient then trying to stack death caps. That 3% boost to AP is so much once you've capped. In addition, it gives you so much mana regen, you can practically keep defile up forever late game. But, I digress, most of the time, games won't last that long.
Some conditional items:
- If the enemy team is particularly tanky, or just magic resist heavy, a straight up 40% magic penetration for a little over 2000 gold is perfect.
- See above. =P
- If you want a little more survivability, and more chase, I suppose you can go with this. I do like it, but to be honest, the straight AP route with Karthus is all I've ever played; I've never grabbed this on him.
- I haven't tried this on Karthus yet, but AP and CDR at a pretty cheap price seem good to me. Testing shall be done. =D
- Only grabbed this once, and it enabled me to solo Baron. I can't say anything bad about it anymore. xD The sheer amount of damage you do with spells will heal you for half your health pretty much every hit. LOL

Items NOT to grab:
- With the play style that's given here (and how I do things), you don't need survivability. You need mana, and AP. Arcchangel's will do just that, and give more AP to boot. And cost less.
- I personally hate snowball items, and this build is too risky and throw-your-bony-face-into-everything to be able to take full advantage of it. I'm not saying it's bad, I'm saying it's not really appropriate for this style of play.

As Karthus, you will always want mid. I honestly don't think there's any champion in LoL as of yet that is better for mid. Your team WILL want your ultimate online as soon as possible. And with the range of Lay Waste, there are few champions that will be able to put up with your harassment, leaving you free to last hit minions as you please.
As soon as you get even the least little bit of AP, you'll be hitting towers like a truck. I'm honestly not sure why this is, but I know that as soon as you get in the 400s and beyond, towers don't last long.
As soon as you think you're done with mid, use this 'glitch' (I've been told it's not a glitch, but I still call it that...being able to six shot an inner tower late game is ridiculous LOL) to your advantage. Help your team push other lanes. Also, as soon as you hit 13 or so, and/or have your rabadon's death cap and maybe another blasting wand, you can solo dragon easily, so don't be afraid to just walk over and do it. Keep up and pop . It'll fall within 6-10 seconds.

And this is where this build goes 'wonky'. I'm going to tell you something you may not believe: I'm telling you to initiate team fights. I'm telling you to turn on and get right in the middle of it. I'm telling you to even in and get in their face. The sheer amount of power in Karthus is enough to merit the lead of the charge. Yes, they will focus you down in a matter of seconds...which is all you need. Team fights normally don't last more than 10-20 seconds, so when you die, you'll have 8 seconds of un-interruptible spell casting and a cloud of death and decay (defile) that your opponents will NOT want to stand in...which is where your team's CC comes in. Your opponent's can't ignore you when you're in the middle of them. But killing you is the last thing they'll want to do, but they will anyways. Utilize to cast your ult to either finish opponents or help your team in the team fight.
I know that last bit may have been a little rant-esque, so I'll sum it up like so: Karthus deals too much damage to sit back and try to snipe with . Get in the thick of it, die, and watch your opponent's cry as you and your team obliterate them. You will die...but when you die, the rest of their team will, too. Karthus is the death singer for a reason.

Some things that I haven't covered that you should definitely keep an eye out for:

Enemy champs that will be a pain:

- I hate to see Veigar. Ever. I try not to play Karthus during any week that he's free. His ultimate ( ) can (and will) one-shot you. Period. You can't really get into good placement for defile and lay waste when you die right on the spot. Keep away from this tiny master of being a pain in the arse.

- He's not so much a pain like Veigar, but being able to flash in, land some good hits and/or kill you, and flash out without taking much defile and lay waste damage make him quite a threat. The only thing I can really say with him is be sure to be able to predict where/when he'll flash and compensate. It'll take some learning, but after you get used to it, you should do okay.

& - They have a natural way to avoid your ultimate ( and ), just be wary of this and try to wail on them when they can't use it. Both have a decent cooldown, so be smart with your spells is all.

Any Champ with a Silence - This annoys me to no end. Any champ with a silence will greatly decrease your dps. In rare cases, they could even make you run out of mana if you have defile up and can't turn it off (that's rare, but it has happened to me before). Also, if you do try to cast your ultimate while you're alive. I don't do it often, but when I do, I make sure that I'm away and safe.

Items to keep an eye out for:

- Dps champs will take hexdrinker to try and counteract your ultimate. If they grab it early game, you probably won't be able to kill them right away. If your teammates call for an ultimate, make sure they know how much it will hit for and if it has proc'ed yet.

- Casters will grab this to be able to survive and ult. Be sure to know when they've used it. Normally, I haven't seen it put to good use (after a team fight, I mean), but knowing who has one is half the battle.

- The most Obvious answer to Karthus, and possibly the WORST answer to Karthus. Yes, I said the worst. In a team fight, that veil won't last long enough to protect from an ultimate. In a one on one, you won't need to cast your ultimate (in most cases...I haven't seen too many champions that can take on a karthus one on one...lay waste + Defile = too much damage). I agree fully with HotShotGG when he says that a tank shouldn't take a banshee's veil...the same reason tanks shouldn't take it is why it's not a good reason to take it to counteract Karthus's .


Another thing to keep in mind is the fact that you'll barely cast your ultimate while you're alive. A Karthus Ultimate hits for too much; you don't want it interrupted. Run into a tower and hit a champion if you have to, as long as you die, you can cast your ult without any interruptions. Just be sure you can kill someone before you throw your face into anything =P
And, I can't stress this enough: BE SMARTLY AGGRESSIVE! People won't expect that from such a squishy champion, but don't go throwing your face into everything blindly. Don't go 1v2 against your opponent's in front of their tower, but go ahead and do it if it's a 2v2, or maybe even a 2v3. As long as you think you can finish off an opponent or help a team mate do so, you'll win in the end by sheer numbers. You may end up with justas many deaths as you do kills, but your team will win overall.

In summary, this guide is meant to shine a new light on Karthus...to try and take advantage of the sheer power and capabilities he has and make your opponents cry. I do have some screen captures of how I've played with him, so let's see if I know how to do it.

They worked! =D
The first picture was just a picture of a game where I was fed so hard...they wouldn't surrender, so they paid the consequences. The second is an example of a ranked game where the opponents were quite magic resist heavy (all of them had a banshee's veil, lol...that made our AD happy), so I took the boots and void to compensate. They caught me farming quite a bit, but I still managed to make quite an impact in the game.

Leave me a comment telling me what you think! And don't be afraid to ask questions about it! I realize that some people may not agree with this build, but don't vote it down because of it! It's just a unique way of playing karthus. =P