Okay so I've been playing Karthus for a long time now and this is the best build I've found for him and i use it time and time again. It's gotten to the point where my friends continuously call for me to play him and i just cant bring myself to do it because I'm bored of winning sooooo much.

Pros

• Huge range! Lay Waste reaches 875 to the center and has a radius of 200 for a total of 1075
• Spam-able DPS. Lay Waste has a 1 second cool down.
• Great CC. Wall of pain slows for 80% movement speed at level 5.
• Sight Ward :D Wall of Pain allows you to see places you would normally be forced to face check.
• Gathers Assists Very Easily. Defile and Requiem make gathering assists a cinch.
• Keeps Mana Fairly Easily. As long as defile isn't being used Karthus will almost always have mana reserves.

Cons

• Squishy! Karthus is not inherently a very healthy character.... being dead and all.... :D
• Highly Feared! Sometimes teams are frightened of your killing power and they will attempt to shut you down early with a team gank. Also can quickly become the main focus of every single team fight.
• Tanky Teams Karthus sometimes has trouble killing when there are no squishy people on the other team.
• Melee DPS's Karthus can sometimes have trouble from characters like Akali, Tryndamere, Sion etc.

SQUISHINESS

1. Buy 9 Health per Level runes.Greater Seal of Vitality Gives an extra 175 hp at lvl 18
2. Buy Rod of Ages. More HP means staying alive longer and sometimes that means all the difference.
3. Do Not Initiate! When you initiate you die simple as that
4. Buy Zhonya's Hourglass. This is only for games when your having trouble late game and just need to survive a little longer.

HIGHLY FEARED

1. Be Prepared For Ganks Play cautiously long enough to account for all enemy champion positions. Junglers obviously make this more difficult so be careful. Be twice as careful if their junlger is Shaco.
2. Do Not Initiate! I told you once. Now i told you twice.

TANKY TEAMS

Magic Resistance?

1. Buy Void Staff. Can you say bye bye Magic Resistance? Of course you can! You're playing Karthus! :D

Tooooo Maaaany Hit Points?

1. Buy Will of the Ancients. Thanks for your hit points suckaaaa.

MELEE DPS'S

1. Exhaust then kill or run. Choice is yours my friend.
2. Wall of Pain slow them down and get away!
3. Defile if they can only keep pace they take damage for chasing and eventually lose interest.

Greater Mark of Insight: Gives 8.5 Magic pen in total. When added to Sorcerer's Shoes this totals a 28.5 magic penetration. Great :)

Greater Seal of VitalityGreater Seal of Vitality: Adds 175 Hp to your late game. Finally! More HP!

Greater Glyph of Force: Gives more AP late game then 3 Quintessences of the same type. Totals 28 Ability Power at level 18

Greater Quint of Potency Gives more AP early game then 9 Glyphs of the same type. Totals 15 Ability Power at level 1

Self Explanatory. Just look at the Mastery Trees

Start With Meki Pendant and dependent on whether you think the champion your laning against will deal lot of damage or not 2 health potions or just one health potion and one mana potion . Choose wisely O.o

Rush the Tear of the Goddess as fast as possible and buy boots if you can! The Tear can take upwards of 30 minutes of game play to max out it's bonus 1000 mana so it's best to get it early. The bonus mana from this item will carry over into a Archangel's Staff , which comes later, and will need every point of that mana possible.

Next buy Mejai's Soulstealer . A must have for any karthus player. Even if your doing badly this item will still collect a good amount of stacks, considering the number of area of affects spells in Karthus's arsenal, and when your doing well..... It's just deadly.

After your Mejai's Rush a Rabadon's Deathcap . The extra AP from this item means business and if you haven't gotten any kills before this, you will now.

Next finish off your Sorcerer's Shoes and buy a Rod of Ages to keep you healthy, alive and give you enough time to max out it's bonus mana before you buy your Archangel's Staff which is next!

Archangel's Staff . If you've done your build right you should see a giiiiiant and I mean GIIIIANT leap in Ability power after purchasing this behemoth of an item. If I've done my homework right, and i think i have you should see an increase of 192 Ability Power! Whoah! That should put you right in the range of 800 ability power! If youve maxed your stacks on Majai's it should be even closer to 1000!!! daaaaaaamn yo. one hit something with your ulitmate! which does around 1100 magic damage to every champion! (give or take : P) Oh yea did i mention with this much ability power your lay waste does around 800 damage to a single target heh heh, heh heh. Go nuts bro.

Last item! Completely situational.

If your dying in team fights and are comfortable with it. Buy a zhonya's hourglass .

If your having trouble against a beefy team that just wont seem to die or your doing really well and just wanna have fun, buy a Will of the Ancients . Oh yea and it helps your team too or something rather. : P

If your having trouble with Magic Resistance. Buy a Void Staff and kill that Galio or whom ever!

***** Currently testing out Lich bane with the new masteries and summoner spells. I'm assuming increased attack speed from Surge will help alot with getting hits in with the lich bane. Karthus has 800-950 ap end game with this build anyways. thats alot of damage am i right :D *****

Look at the Sequence shown above! I refuse to write anything more about this!

**** To be updated once i get the hang ot the new play style and spells *


Exhaust:

SOOOOOOOOOOOOOOOOO goood! You know those times when you just cant outrun master yi or someone of the like and your like ohhhh daaaamn im so squishy how will i ever survive this fight!! Thats when you use exhaust! Soooooooooo many times have i killed the main enemy DPS because he thought he could 1v1 me if he closed the gap quick enough and its alllll because of exhaust! Eg. (This story is brought to you by real life events) do da dooo just walking along the bushes when BAM!!! xin flys out at me with his lunge move. Bam! Defile! Exhaust! Lay Waste! Lay Waste! Lay Waste! Oh no you dont where you goin son! Wall of Pain! Lay Waste! Dead xin! Heh Heh Heh teach you son.


Flash:

Cause a escaping is nice and a flash exhaust lay waste kill is just ohhhhh sooo nice early game in the middle. (Pay attention to your enemey's Summoner spells!)

Grab blue! like all the time! Even if someone else wants it hehehe XD Endless Defile = win. Endless wall of pain = Win faster.

Blue can usually be taken very quickly around level 6-7 shouldn't need too much mana to accomplish this either 1/5-1/4 should be enough. after getting blue (If you have enough health) make sure to pick up dragon as well. It's pretty easy for Karthus. He can solo and quickly at level 9.

Baron shouldn't be attempted solo. Though if you know the teams taking him walk over and Place your wall of pain in there. With a well placed Lay Waste you can steal that baron pretty easy. I've done it to dragon and I've done it to Baron countless times!

MANA

Defile drains ALOT! of mana! only use it sparingly until late game or for when you have blue buff. If your not careful with Defile you can find yourself in a fight with no mana and thats not a good thing. Sometimes you'll find yourself wishing you had enough mana to drop just one more Lay Waste for the kill but you've already wasted it all on a relatively undeveloped Defile . Or even worse no mana to cast Requiem and a hero makes it back to base too quickly.

A good way to keep mana during lane time is, conveniently enough, last hitting and killing creeps. Alot of times during play i will pop my Lay Waste on the back 3 minion casters until i kill all 3. I do this for 2 reasons. 1 upon killing all three (sometimes in 3 sometimes in 2 castings) I am awarded mana for all 3 kills by Defile . The second reason is to ensure an open area where i can hit the enemy champion with Lay Wastes double damage effect.

The last thing i have to say about mana is just a simple trick, when you run up to a large group of creeps with Defile on to destroy them quickly. Turn it off just before killing the group and finish them off with a Lay Waste or two. This'll give you a return on some of that mana you just spent instead of relying solely on your mana regeneration.