Hi, I'm Genje, and welcome to my Karthus guide. This is the first guide I've created on Mobafire, so please let me know what you think and give me some feedback in the comments. Keep in mind this guide is assuming that you are going middle lane in 5v5 Summoner's Rift. Without further ado, let me show you all how I play Karthus.

Pros + Great at last-hitting + Built-in sustain + Great at harassing + Amazing passive - kill enemies after you die + Kite enemies all day long + Global Ultimate

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Cons - Squishy - Tends to be focused in team fights - Skill-shot based - Easy to counter

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It pretty much comes to down to individual preference for choosing which mastery page you want to use. You could make your own mastery page as well, I just find these two to work very well.

Greater Mark of Magic Penetration: These are your best choice of marks. These runes allow you to ignore 8.5 of your enemy's magic resistance, causing them to take more damage from your spells.
Greater Seal of Scaling Mana Regeneration: These give much needed mana regeneration during the mid and late game.
Greater Glyph of Scaling Ability Power: Adds nice bonus damage to your spells which increases with your champion level.
Greater Quintessence of Ability Power: Arguably the most powerful quints for AP casters. Adds about 15 AP starting from Level 1, making your early game spells deal more damage. This allows you to both last hit and harass better during the early part of the game.

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Greater Seal of Scaling Ability Power: These give you extra AP which scale throughout the game, just not as much as Greater Glyph of Scaling Ability Power.
Greater Glyph of Ability Power: Adds a decent amount of damage to your early game.
Greater Glyph of Magic Resist: Since you'll most likely be facing another AP carry if you go to mid lane, these glyphs will lower the damage you take from his/her spells.
Greater Quintessence of Health: These boost your health starting from level 1, giving you better protection early game.

Teleport: This spell has many great uses. During laning phase, after you recall to buy items, you can immediately teleport back to your lane, and not miss out on any farm or experience. If an enemy is pushing one of the lanes to your tower and none of your teammates can get there in time, teleport to your tower to save it. If enemy champions in the side lanes are overextending or in a close fight with your allies, teleport to a nearby allied minion or ward. This will turn the fight into a 3v2 or 2v1 situation, winning the fight.

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Flash: A great spell for increasing your mobility. Flash forward to surprise an enemy or kill someone just out of reach. Flash toward to your tower or team to escape a gank/bad positioning. This is especially helpful combined with your Wall of Pain to escape. It can be used to jump over most walls. If you're about to die and your Flash is up, Flash into the middle of the enemy team right before you die. This will allow you to deal lots of damage and assist your team after you die.

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Ignite is usually used to kill an enemy who is at very low health. This is a good spell to take if you know that there are champions on the enemy team that can quickly regenerate their health, such as Dr. Mundo.
Ghost is another good spell. It can be used similar to flash to either catch up to or escape from an enemy.
Revive is very powerful when combined with Teleport to re-enter a fight with extra hp and a full mana bar after you die. I personally haven't tried this, but I've seen other Karthus players use it very well. Also, if an enemy gets to low health and you are dead, just Revive and press R. ;)
Clarity replenishes your mana, but usually the passive on your Defile should be enough along with your runes/masteries to be fine if you are last hitting minions.
Heal is good for surviving ganks or fighting in a 1v1 battle. I feel there might be a few more powerful summoner spells though.
Smite is not useful unless you're going jungle Karthus. Jungle Karthus isn't covered in this guide, but with the new changes to the jungle it's viable.
Cleanse could be very useful if the enemy team has a lot of crowd control abilites.

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Surge is a spell that increases your AP for a short time, which isn't bad, but there are better options with your limited amount of summoner spell slots.
Garrison Useful on Dominion, but it isn't supported in Classic mode.
Exhaust isn't worth all that much to Karthus. He already has his Wall of Pain.
Clairvoyance is nice, but is usually left to support champions. Karthus has better options.
Promote isn't really that useful in my opinion. It doesn't take much for the enemy to kill your promoted minion.

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Death Defied (passive): This passive activates immediately after you die, and can be extremely powerful if used correctly. Hopefully you won't have to use it too often, but if you do, just plop down Wall of Pain, turn on your Defile and spam your Lay Waste. You can get assists this way and sometimes even a few kills. As soon as you die though, you will be unable to move or use any summoner spells.

Requiem takes 3 seconds to channel, so if you want to use it after you die, you only have 4 seconds to cast your other spells before you must channeling Requiem. It takes practice to get the timing right, but this usually means you only have time to cast your Wall of Pain and a couple Lay Waste. Nothing is worse than waiting too long to cast your ultimate and missing a great opportunity to use it.

• If you know that you are going to die, walk into the enemy team before you do to maximize the damage you do after you die.
• During Death Defied you are untargetable. This is a good time to use your Requiem because it cannot be interrupted by the other team.

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Lay Waste (Q): This skill will be your main source of damage. It is also your main harassing/farming skill.

It has a .5 second delay from when you press Q to when the spell actually takes effect. This means that you must lead the target.

• Lay Waste has a small aoe, but it deals double damage if it only hits a single target. Use this to your advantage when last-hitting minions and attacking enemy champions.
• Once you buy Rylai's Crystal Scepter all of your Lay Waste attacks slow the target if they hit. Use this to chase down escaping enemies and to escape. If you are being chased and have Rylai's Crystal Scepter, simply Lay Waste the chaser and laugh as you walk away. Be careful of champions with some type of cleanse or that can avoid being slowed, like Master Yi.
• Lay Waste reveals the fog of war. Use it to check bushes.

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Wall of Pain (W): This is your only built-in form of crowd control, but it is extremely powerful. Not only does it lower the magic resist and armor all enemies that pass through, but it drastically decreases their movement speed as well.

• If an enemy touches Wall of Pain at all the debuffs will proc. This allows to directly place the wall right on top of an enemy to debuff them.
• Wall of Pain works extremely well combined with your Lay Waste. After you slow an enemy champion with Wall of Pain, your Lay Waste will be easier to hit because of the slow and deal extra damage from their reduced magic resistance.
• Wall of Pain can be used both offensively to chase or defensively to slow pursuers.
• When ganking a side lane, come up from behind and lay down Wall of Pain directly in their escape path.
• Along with Lay Waste, this spell reveals the fog of war. Only the "poles", or the ends of the wall, will do this though.

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Defile (E): This ability is very similar to fiddlesticks' ultimate, but it has a few other benefits as well. It can be run indefinitely until you run out of mana. Also, it has a passive which gives you mana every time you kill a unit.

The damage over time when Defile is activated is not even. It deals damage in 1 second "ticks".

• Last-hit minions not only to farm, but to use Defile's passive to its fullest. Leveling this ability will give you most of the mana sustain you need to stay in your lane for long periods of time.
• Don't activate this ability often in the early game, because it will drain mana too quickly.
• Middle to late game, Defile can be used to farm minions much faster than Lay Waste.
• If you activated Defile before using the active from Zhonya's Hourglass, your AOE damage will continue ticking.

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Requiem (R): Here it is: The Ultimate. This ability is what makes people hate Karthus so much. It deals damage to all enemy champions on the map regardless of distance. It is usually used to finish off a low health opponent who barely escaped a fight, but it has many other uses too.

When trying to finish off an enemy champion, several things must be taken into account before Requiem is used. Does the target have a spell shield, a quick way to heal, or have an ability to dodge your ult? You must also factor in how much magic resistance they have into your calculation. The more games you play with Karthus though, the easier it will be to decide if it's the right time to use your ultimate or not.

• If one your allies is in a very close fight with an enemy champion, use your ult to give your teammate the advantage needed to win the fight.
• Requiem can be used during Death Defied. This is an ideal time to use it because it cannot be interrupted.

Meki Pendant +2 Health Potions is what I normally start with. The meki pendant gives me extra mana to use Lay Waste and 2 health potions to heal from any damage you take.
Boots +3 Health Potions is also a another possible start. The Boots allow you to dodge enemy abilities easier. Useful if you know that you're going up against someone with a lot of harassment.

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I like to get Tear of the Goddess right after Boots so that I can start getting the bonus mana, as well as the mana regen, as soon as possible. This will be built into an Archangel's Staff much later in the game. These are standard AP caster boots. The magic penetration is good for increasing the damage of your spells. You can switch these boots with Ionian Boots of Lucidity for more cooldown on your Requiem after you get a Void Staff. The ability power is nice, but the main reason for buying this item is the spell vamp, which is great on Karthus, or any spellcaster. This item is a wonderful multi-purpose item. Alternatively, you could buy a Rod of Ages as your defensive health item, but I've found that the slow from Rylai's Crystal Scepter helps me perform much better in game by slowing enemies when my Wall of Pain is on cooldown.

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A staple item for every AP caster. After you finish this, you'll see a huge boost in your AP and damage. After you get Rabadon's Deathcap, you'll be doing enough damage to justify getting some more spell vamp. By now, the bonus mana from Tear of the Goddess will be very high. By getting an Archangel's Staff, you'll convert that mana into AP. The mana regeneration on Archangel's Staff will also be extremely useful.
This item is great when the enemy is building magic resistance against you. Feel free to add this item earlier in the build if they start stacking magic resistance.

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The armor is very useful if they enemy team's ad carry is getting fed. The active is extremely nice too, and can be used in conjunction with Defile to deal free damage to the enemy team. This item is useful if the other team is mostly AP-based because of the magic resistance it provides. Another good item against AP carries because it automatically blocks one spell. If you are up against a lot of harassment early game, you could rush a catalyst the protector instead of a Tear of the Goddess and build it into a RoA. If you are crushing your lane really hard early in the game, then this could be a good purchase. When you get stacks though, you must try especially hard not to die.

• During levels 1-5 concentrate on last hitting minions with your Lay Waste.
• Harass your opponent when you can, if you feel it is safe and if you won't miss out on killing enemy minions by doing so. When you do harass, try to "feel out" your opponent. If your opponent is usually aggressive when you get close, aim your Lay Waste in front of them. If they back off, aim your Lay Waste behind them.
• Try not to let the lane get pushed in either direction. If you push too far, you could get ganked very easily, while if the enemy pushes too far, your tower will take all of your enemy minion kills. Remember to call "MIA" if your opponent goes missing.
• If you're looking to finish off your opponent or if your jungler is looking gank your lane when you're level 4 or higher, initiate the fight with a Wall of Pain.
• Don't activate Defile until higher levels unless your mana bar is nearly full, as it drains mana extremely fast during the early/mid game.

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• When you reach level 6, immediately step back in your lane and look around the map for any opportunities to use your Requiem. If an enemy champion has 2 HP bars or less, you can probably get a kill.
• Grab the blue buff from your jungler whenever you can.
• Keep a look out for any close fights that you could teleport into to secure some kills and possibly save an ally's life.
• If your team is fighting for an objective, such as the dragon or blue buff, make sure you are there to contribute.

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• During team fights, try to position yourself to where you can deal damage but not get focused. Karthus is squishy, and will die very quickly if you are focused down by the enemy team.
• Always focus on killing the squishiest people on the enemy team unless someone else is at low health, then focus on them.
• If you feel that your team is losing a teamfight, stay near the edge of the fight so you can retreat if needed. If your team is winning, you can get more aggressive. Use Defile when any enemy champion is in range, unless your mana is low.
• Right before a teamfight starts, you can use Requiem to weaken the entire enemy team. This is when it will deal the most damage, because the whole enemy team is still alive.
• If you realize during a fight that you are going to die no matter what, walk into the center of the fight with Defile turned on, so you can maximize the damage you do after you die.

Here are just a couple screenshots of some good games that I've had using this build.

6/19/12 - Changed recommended mastery from Swiftness to Improved Recall . Changed recommended seals from greater seal of vitality to Greater Seal of Scaling Mana Regeneration, and starting item from Sapphire Crystal to Meki Pendant. Fixed some typos, and added a few bits of information.
6/20/12 - Added a Gameplay section.
7/7/12 - Fixed some more typos.

Thanks for reading my guide on Karthus and thanks to JhoiJhoi for making her Guide to Making a Guide, which was extremely helpful. If you had any suggestions or feedback just post a comment.