This is a build made for katarina AP/assasin style mainly focused on quick gettaways, low cooldowns and high spell damage. I made this build with help from Janitsu as i'm not really used to play with katarina so i would like to thank him for testing it ^^

The build is designed for 5 vs 5 use! it is not been tested for 3 vs 3 and i doubt it would work there so do not use it in 3 vs 3 unless you want to test it.

update v1: after interesting comments i decided to change a few things as they could be replaced with better items and i changed the mastery tree and quints a little.

For runes i decided to mainly focus on 3 most usefull aspects namely cooldown reduction, magic penetration and ability power. The ability power helps early game and is a bonus later on. Cooldowns also get better each level making your skills faster and faster. Magic penetration is usefull as you'll deal more damage through their magic resist early game.

For masteries i decided to go for both utility and offensive as they both lower cooldowns. Offensive side gives me more AP, cooldown reduction and magic penetration. The utility side gives me a small speed boost, also cooldown reduction, more experience gain which can help getting the upper hand, longer duration of monster buffs and less time spend dead. The last few points i give to my summoner spells to increase their effect or lower their cooldown.

I would start with afor increased damage with your shunpo from start aswell as a usefull item to upgrade from. Now we can start fighting while using a shunpo - flash combo for finishing of retreating opponents with low health close to tower. Once you got enough gold you can either upgrade your tome toor go for thedepending on the amount of gold you got. Your next objective is to get the other one of the two you picked. Now your spells will heal you for an amount of money making your chances of survival bigger. Now you can either upgrade your revolver intoordepending on your team. Pick the first one if you're teamed with ap nukers or pick the second one if you're teamed with close combat champs as you can slow your opponents with it making them an easy prey for your team. With these items ready you can go around easier killing off the weaker opponents or assisting your team when necessary. Now we go for theand theto finish things off making you very usefull as your spells can slow multiple enemies and deal quite alot of damage. Now we can finish our build with a range of items depending on the opponents you play against. I usually would take themaking your spells having 50% magic penetration which is usefull against tanks. As last item i would go forfor increased armor , more Ap and a very dangerous active if combined with a well timed death lotus.

flash: usefull for escaping when shunpo is on cooldown or usefull for the tower diving combo to finish off opponents.
ignite: usefull to finish things off when your opponent is getting out of reach after they took massive damage from your spells.

other usefull spells: ghost, cleans, exhaust

pro's:
- usefull multislow spells late game
- deadly ulti for multiple targets
- fast with the shunpo-flash combo
- can farm pretty fast
con's:
- first target when visible in team fights
- vulnerable to disables
- death lotus isn't invincible, you can be disabled easily if you rush things by doing it to soon.

I'd like to end my build with some extra tips while playing katarina with this build:

- stay out of sight in team battles and find the right moment (after all disables being used) to appear with shunpo and death lotus to give your team the upper hand or to change the tables around.

- do not rush things, every time you pick a target, wait for the right moment and then strike the deadly blow just like an assasin does before fading away into the shadows again.

- never take the lead in battle unless you're team is skilled enough to see a decoy strategy and if you're skilled enough to pull it off being the decoy(= only for advanced users, if you don't know what i mean by this strategy then do NOT try it)

- Use killer's insticts before using Shunpo to get near enemy. Now you'll take reduced damage and Death Lotus deals alot of damage.

- use bouncing blades and shunpo for farming minions fast when your opponent has left your lane or when they are dead. Both skills have multiple targets and deal all targets enough damage to finish them off.

This concludes my build about katarina AP/assasin style, i hope its been usefull to you all. If you have any questions feel free to ask or if theres any feedback you'd like to give feel free to post below at the comments ^^