Runes

The runes I run on Katarina are typical of some AP mid assassin's and are similarly used among champions like: Fizz, LeBlanc, and Kassadin. However this rune page in general isn't the bread and butter page of these champions and should always be considered situational at best. I run this particular page for it's mid/late game strength and lane survivability.
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Runes

Precision

Fleet Footwork

Phase Rush

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• Greater Quintessence of Ability Power : Are the standard AP mid quints, they provide the most AP for early game and allow for your Bouncing Blade to become greater tools for early game farming and harass, without them your next to useless in the lane. Essential.
• Greater Mark of Magic Penetration : Also essential to you harass and farm, as well as your ability to penetrate your enemies magic resistance early/mid game. Don't leave the platform without these!
• Greater Seal of Magic Resist : These can be swapped with Greater Seal of Scaling Ability Power depending upon the demand. Remember that when it comes to Magic related runes Glyphs hold the most capacity, both in damage and resistance. That being said you will need to change these Seals out with the seals of Force when you intend to use more magic resistance in the lane. Opponents known to run flat AP or opposing champions with high AP ratio's should be the only people who would warrant you to change your Seals.
• Greater Glyph of Scaling Ability Power : Take these to maximize your damage potential mid/late game. As this will be the time were your toolkit will be most essential these Glyph's become most essential. As mentioned above can be changed out for Greater Glyph of Magic Resist for more magic resistance.

Masteries

My choice of masteries is again typical of most AP mid's especially those who aren't dependent on lots of mana. Since Katarina is a manaless champion avoiding the Utility tree allows her to maximize her sustainability and damage potential both in lane and during teamfight's.
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• : Summoner's Wrath simply put it makes your ignite slightly more useful. It also helps enhance exhaust should you need that.
  
• : Mental Force 3 ability power is much better than 3 attack damage. This is debatable as some would say she does need to auto attack but consuming the marks off her Bouncing Blade provides enough of a damage proc. to farm.
  
• : Sorcery essential to acquire the Arcane Knowledge perk. Plus 4% cooldown reduction isn't bad on Katarina.
  
• : Arcane Knowledge is among the best of the offensive mastery tree. +10% magic penetration exponentially enhances your damage.
  
• : Blast 18 ability power at level 18, one of the best things you could have since you will want all the ability power you can get. This combined with your Greater Glyph of Scaling Ability Power will give you 23 ability power at level 9. Essential for your roaming strength mid game.
  
• : Archmage is great for increasing your overall lategame AP.
  
• : Executioner allows for your entire role to be performed smoother late game. As it will assist you in bursting down pesky near death enemies and keep your passive activated with each kill.
  
• : Resistance is just a no brainer as you will be mid verses an ability power champion. Most of the time...
  
• : Durability allows you to be a bit more tanky, since you will be in the middle of things this can be very helpful.
  
• : Veteran's Scars gives you the early game health to stand up at melee range to your opposing laner while your Bouncing Blade are still under leveled.

Itemization and Rationalization Early/Mid Game

Item Sequence

Health Potion 50

Amplifying Tome 400

Stealth Ward 0

Stealth Ward 0

Frostfang 400

Hextech Revolver 1050

Rabadon's Deathcap 3500

• : you want Boots and three health pots early on because it will help you last hit your minions with speed since your Bouncing Blade won't reliably farm the lane early on.
  
• : You want the Amplifying Tome early on because of it's base AP and the fact that it will soon be built into a kage's lucky pick. However, it's importance as an item varies from lane to lane as many would simply say to just wait until you can afford the kage's all together. However my response to that is below.
  
• sight ward: This is a general rule of thumb for most mid's. Always maintain your two wards in the brush. Sight is paramount to survival so I often wait until I have enough gold for Amplifying Tome and 2x Sight Ward's, this generally happens around lvl 5 which I find helps me prep for the possibility of lvl 6 gank's from the jungler.
  
• Kage's Lucky Pick: It's simply a must have for me. Some people opt to transform it into a fast Deathfire Grasp but I love to keep it around for it's passive gold gain and cost effectiveness. However if you feel it dumb you can always supplement it for components of the Hextech Revolver.
  
• : Sustain is essential to Katarina. Although when you have enough AP and lane presence you can simply Bouncing Blade a minion, hop to it, and W the wave and leave early on that cannot be achieved against specific laners. As such the spell vamp from the Hextech Revolver allows you to take some poking while farming at melee range on occasion.
  
• : Simply foolish not to buy it. As I prefer Katarina to be played like an assassin I opt to buy this as my first major item for large early/mid game burst potential.

Itemization and Rationalization Mid/Late Game

Item Sequence

Rylai's Crystal Scepter 2600

Void Staff 3000

Zhonya's Hourglass 3250

• : you want Will of the Ancients for it's additional spell vamp and team synergy. Because your likely to be focused late game your team should likely run a synergized team composition utilizing another champion such as: Amumu, Rumble, Kennen, Malphite, etc. This item will allow for not only yourself to be more sustainable during your aoe ult but your synergized teammates as well.
  
• : This item is often considered to be the standard defensive item slot holder. It is often interchangeable with other situational items listed in below in this chapter. This slots function is just that, defense. Since Shunpo, and Death Lotus will stick you well within melee range of an entire enemy team you will need to bring some tank to the table otherwise you will be shredded. A lot of the time this can be prevented with proper team synergy but you can never account for the Quicksilver Sash or Cleanse to be on another opposing carry.
  
• : A late game AP carry item intended to penetrate the Magic Resistance that opposing players have built or spec. into. You can supplement this for virtually any item of your choosing if you feel the need. However, keep in mind snowballing often makes many AP assassins over confident in their damage tradeoff and they seldom ever buy this item. Allowing teams with true tanks to capture, absorb, and demolish you since you can no longer kill them. Keep that in mind.
  
• : Although this does stop you ultimate its intention is for resetting team fight's, surviving grabs/initiates, and prolonging the fights. It's high Ability Power and Armor make it a great overall item for any late game AP carry.

Situational Items

Item Sequence

Hextech Gunblade 3000

Frozen Heart 2500

Guardian Angel 3200

• : Interchangeable with the Rylai's Crystal Scepter item above. Another Defensive item that you may be prone to using on katarina late to mid game. I prefer to build this item against high AP team compositions who may be running an additional AP champion top lane like Vladimir, Rumble, etc.
  
• : Hextech Gunblade is a great item to have on top of the Will of the Ancients as the multiple spell vamp will make you very sustainable during your combo and your ult. I only tend to buy this if the enemy team has a lot of damage over time abilities or if I'm stuck in a difficult lane transitioning into the late game.
  
• : FH is great for those two or more ad team comps that have a strong melee presence. Since your often forced to go toe to toe with them this item becomes essential in reducing their damage output as well as the frequency of their attacks. The CDR also improves your ability to combo faster.
  
• : If your getting focused a ton during fights and you have reached a high carry level buy this item. It will give you the second wind you need to take down the enemy team long after they've wasted all their cooldown's on you.

Abilities and Explanations

• : Voracity is Katarina's passive ability. It effectively allows you to completely refresh the cooldown's on all of your abilities save for your Death Lotus after killing an enemy. This will allow you to chain multiple kills together in swift succession.

• : Bouncing Blades is Katarina's bread and butter ability. You use it for farming and for harassing enemy laners. Easy blade leaves behind a mark that can be "consumed" via auto attack or an additional ability. This mark provides additional damage. Max this first.

• Sinister Steel: Sinister Steel Essentially allows you to harass the hell out of skill shot based carries and enemy champs in general. Using this ability on champions grants great additional movement speed that allows for fast wave clearing, swift harassment, and the ability to narrowly escape ganks, cc, and teamfights. Max this second.

• : Shunpo Is Katarina's gap closer. This ability provides additional damage and a means to catch up to an enemy effected with a Bouncing Blade mark and consume it. Then active your sinister steel to kite and evade with the additional movement speed provided.

• : Death Lotus is Katarina's main source of chunk damage. It is best used in synergy with AOE stun's. It can be canceled by any form of cc.