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Katarina is a snowball cleanup champion. She could be called the "Janitor", cleaning up every fight, and deals much damage in fights.

The problem with Katarina is she can never really be balanced because she either gets too far ahead and reset pentakills people or she's useless.

Quick tips: There's a few things you can do with Katarina. The most obvious one is using shunpo (E) to wards/Jarvan flag/Thresh lantern to be able to connect distances very quickly. The only unique trick that's interesting is when you shunpo on an enemy ward, you can make it stay alive for another 1.5s~ which can allow you to single handedly kill wards by doing auto attack -> shunpo -> auto attack -> auto attack.

Unless you're against a very potent assassin, an AP top/mid duo or jungle/mid duo, I largely prefer:

Quintessences (x3) can be movespeed or AP flats. It's entirely situational and I end up using either page pretty frequently. Movespeed allows you higher mobility off her already very high base movespeed. This allows you to space better in lane and approach team fights/roam more effectively. AP quints are just more damage,and I'vealready explained why damage is extremely important on this champ.
AP/LVL (scaling) glyphs (x9) because she has magic resist scaling per level and starts with extra magic resist from defense masteries. I feel like she's a champion that needs to maximize her damage to play effectively and a lot oftimes that extra damage is the hit or miss factor for a triple kill from your reset passive.

Hybrid penetration or magic penetration for marks (x9), depending on matchup or preference. Hybrid is more useful against melee champions that you can actually get auto harass on but kat's early game is so pitiful that a lot ofgames end without you doing much auto damage at all during the laning phase.
Armor seals (x4), it's standard to have some armor.
Magic Recist (5x),

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Maxing R->Q->W->E is good against champions who are ranged and hard to commit on to. A good example is Orianna because she's a champion where you're going to need to harass down from range before being able to commit for an all-in.

Alternative: Another variant is maxing W for more consistent melee dps but not much ranged poke damage. Typically going Q W E W W R W Maxing W->Q->E is only possible against melee mids such as Gragas. It definitely helps your mobility though because the mini speed-boost allows you to chase and space well.

Voracity:

Scoring a kill or assist will reduce all the cooldowns of Katarina's abilities by 15 seconds.
Voracity is an effect that triggers whenever Katarina kills an enemy champion or assists in the death of one.
Additional Information:

Voracity can continue to function and reduce Katarina's cooldowns even after she has died.


RANGE: 675 COOLDOWN: 10 / 9.5 / 9 / 8.5 / 8

ACTIVE: Katarina throws a dagger that bounces quickly from enemy to enemy, dealing magic damage and marking each enemy hit for 4 seconds. Bouncing Blades hits a total of 5 unique targets (4 bounces), each striking the closest next target. Each subsequent hit deals 10% less damage.
Striking a marked target with an ability or basic attack will consume the mark to deal additional magic damage.

DAGGER MAGIC DAMAGE: 60 / 85 / 110 / 135 / 160 (+ 45% AP)
MARK MAGIC DAMAGE: 15 / 30 / 45 / 60 / 75 (+ 15% AP)
TOTAL DAMAGE: 75 / 115 / 155 / 195 / 235 (+ 60% AP)

Bouncing Blades is a targeted chain area of effect ability.
Applies spell effects as an area of effect ability.
Spell vamp is reduced to one-third effectiveness.
Rylai's Crystal Scepter will apply a 15% slow.
Magic damage will be mitigated by Black Shield and Lifeline.
Banshee's Veil (All spellshields) will block the ability and prevent any additional bounces from occuring.

Additional Information:

Bouncing Blades can hit targets obscured by the fog of war or who are stealthed.
The mark damage from Bouncing Blades can be triggered and refreshed by other Bouncing Blades.

sinister steelSinister Steel: RANGE: 375 COOLDOWN: 4s
Sinister Steel
ACTIVE: Katarina whirls her daggers, instantly dealing magic damage to all nearby enemies. If she damages an enemy champion, she gains movement speed for 1 second.
MAGIC DAMAGE: 40 / 75 / 110 / 145 / 180 (+ 25% AP) (+ 60% bonus AD)
MOVEMENT SPEED: 15 / 20 / 25 / 30 35%

Additional Information:

Sinister Steel has no cast time and does not interrupt Katarina's previous orders.

Shunpo: RANGE: 700 COOLDOWN: 12 / 10.5 / 9 / 7.5 / 6

ACTIVE: Katarina teleports to target unit and gains 15% damage reduction for 1.5 seconds. If the target is an enemy, the target takes magic damage.
MAGIC DAMAGE: 60 / 85 / 110 / 135 / 160 (+ 40% AP)

Additional Information:

Katarina will automatically attempt to attack the target of Shunpo after blinking.
Shunpo can be used on any targetable allied or enemy unit except for towers and inhibitors. This can include champions, minions, wards and champion pets such as Teemo's Noxious Trap, Jarvan IV's Demacian Standard and Thresh's Dark Passage.

Death Lotus: RANGE: 550 COOLDOWN: 60 / 52.5 / 45
ACTIVE: Katarina rapidly spins in place and channels for 2.5 seconds, throwing a barrage of up to 30 daggers at the closest 3 enemy champions within range. Each dagger deals magic damage and applies Grievous Wounds for 3 seconds, reducing regeneration and incoming healing by 50%.
Death Lotus can hit a single enemy champion no more than 10 times.

MAGIC DAMAGE PER DAGGER: 40 / 57.5 / 75 (+ 25% AP) (+ 37.5% bonus AD)
MAXIMUM DAMAGE TO EACH ENEMY: 400 / 575 / 750 (+ 250% AP) (+ 375% bonus AD)

Additional Information:

Death Lotus cannot be activated unless there is at least one enemy champion in range of the ability. Leaving the ability radius while she is channeling will not cause her to stop prematurely.
Death Lotus can be activated even when Katarina does not have sight of her target (through fog of war or stealth effects).

Kat actually has a wide diversity of items she can go.

SEE LIST ABOVE

Pros: she's a very fun assassin that plays at doing damage when ahead and flickering around the map quickly.
Cons: she's an extremely weak mid laner and has only bad matchups in lane. She is hard to gank for and brings nothing outside of damage to a team (no cc). She cannot collapse for jungle invades or really pressure before getting items and levels.

I like this champion because she's fun to play. However, I admit she's a very weak mid lane champion right now especially after multiple base damage nerfs.