Hello guys, this is my first guide and I decided to publish it because I played Kha'Zix on PBE and wish to share my thoughts on him publishing this guide on release :3
I will not spam a lot of lore content here, but just what I think is good or bad on him. I apologize for my english, and every criticism is very welcome on the comments of this guide.

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Pros + Excellent burst damage + Unbelievable juking potential + Kill secure ganks post-6 + Many ways to master skills offer versatility + Scyther comes from da Void to kill some Rengars

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Cons - Squishy (because he's an assassin) - Is an assassin, does not fit the current meta - Somewhat hard to play (since it's an assassin) - Shut down easily - Loses his head to Rengar

Flash. Just Flash. I love this spell, as it provides an excellent escape mechanism, great juking potential and may also (but is NOT the objective) secure a kill. If you don't like it, take Ghost in lane, because you NEED an escape mechanism, and Ghost or Exhaust in the jungle for better ganking.
On a lane, you can choose Exhaust or Ignite for offensive spell, personal choice, but do not take Exhaust if your jungler already have.
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DO NOT JUNGLE WITHOUT THIS ONE!

Default runes, indeed. Greater Seal of Armor and Greater Glyph of Scaling Magic Resist are the most standard rune choices, Greater Quintessence of Movement Speed better choice for assassin too. Greater Mark of Attack Damage helps a lot with last hitting, but greater mark of desolation is better for harassing early on. If you are going mid, Greater Glyph of Magic Resist can be a better choice against a very bully mage. Same thing, just change the Greater Mark of Attack Damage for the greater mark of desolation because you don't need last hitting. Greater Mark of Attack Speed helps in the jungle but are not needed at all.

Taste Their Fear on lane will make you harass and exchange better and Void Spike will increase your health sustainability, in cost of some mana (you can level it up early if you want to, personal choice again). Leap is an amazing skill, but just one point early because it's an utility one. Same as before, just some early points on Void Spike to increase your sustainability. You can skip this to max Taste Their Fear before or just max Void Spike straight if you're not having mana issues. Thanks for SerDigBick for let me know Void Spike is better to first clear.

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Here are some explanation and tricks to get with Kha'Zix toolkit.
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• This passive is really good, because of the slow. But don't forget the damage when you're laning, mainly top
• The passive goes up when you're not in the vision of the enemy team (AKA you're not on their minimap), and is spent ONLY when you basic attack a champion or use your evolved Void Spike
• Walking off a bush to attack a champion, and going back to bush readies the passive again, so try to exchange near bushes
• If you walk into a bush and doesn't have the passive up, it will go up UNLESS the bush is warded.
• Activating Void Assault will make the passive go up, even if there's a pink ward or oracle's on the enemy team.

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• Does not proc on-hit effects, but cooldown is low enough to keep the Sheen proc up.
• Not only increases in range when evolved, but also increases basic attacks range!
• Great finisher on isolated targets if evolved
• With the Mecha skin, it's SO EASY to say which target is isolated
• Don't think you do extra damage on isolated targets, ONLY this skill have this effect
• Works on jungle camps if they are alone (kill little lizards near blue and red to use Taste Their Fear, also works on Dragon and Baron Nashoor

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• Don't use to harass, too much mana for low damage. Most important part is the healing effect
• When evolved, the aim is a REAL skillshot of three projectiles, unlike Twisted Fate's Wild Cards

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• One of better jumping skills ever, with range and AoE like Tristana's Rocket Jump, resets on kill and assist, have a low cast time and scales with AD

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• THE MAIN USE FOR THIS SKILL IS TO KEEP YOUR PASSIVE UP
• It's not a stealth system
• However, one second CAN BE enough to juke, but not to hide
• See Juking section for more detailed information

Okay, here's where everything is settle. The most important part of a build is of course your itemization. Just a comment before we start:
Phage IS A MUST ON KHA'ZIX.
Now I'm ready to start.
. It's the best option to start, for top, mid or jungle. Boots provide a flat amount of MS (50 tier 1, 70 tier 2), and every % of MS is applied on this flat amount. E.g.: Kha'Zix has 325MS, Greater Quintessence of Movement Speedx3 + Swiftness x4 = 6.5% MS, 346MS without boots. (325+50)*1.065 = 399MS. With almost 400MS level 1, you'll just go to your opponent dodging his skillshots, land Taste Their Fear on him and come back without any retaliation, except if he decide to use a gap closer. Definitely worth it.
If you are very scary of the jungle, grabbing a Cloth Armor and 5 Health Potion is not a bad idea either, but will slow your clear and reduce your ganking potential.

. Phage. Phage. It gives you health for cure some of your squishiness, attack damage to increase your burst and a superb passive to make sure a gank will be succeed, even with you are at lane. Heart of Gold gives you some more health and more important, is a GP10 item. You'll probably be bullied top lane because of your squishiness, and in mid lane because of your range, so this is a must. Mercury's Treads is just overpower, tenacity is a gamebreaking stat and you don't want to miss it, do you? This boots is so powerful, except if the enemy team has NO CC, and it's impossible to happen. Take Ionian Boots of Lucidity if you don't like this one, or Mobility Boots for increased ganking potential (both jungling or laning). Ninja Tabi is also a good choice for laning phase against AD, can be changed for Mercury's Treads later. In the jungle, you'll not have that amount of gold, so rush Heart of Gold before Phage. You may also want a Wriggle's Lantern for extra clear speed and sustainability.

. Most games will end like this, because it's a bit expensive build. Bloodthirster is another overpowered item, giving amazing 100 AD in a single slot! And it's stacks are easy to keep up. Trinity Force is a better placement for you Phage than the Frozen Mallet, because your Taste Their Fear can keep the Sheen proc always available, making your damage increase drastically. If you are jungling, also add the Wriggle's Lantern on the core, and just sell it for your LAST item.

. After the core build, everything will be situational, but keeps in mind, Last Whisper is always a good choice. Finishing your Randuin's Omen will make your more durable and the active is one of the bests in the game. This item should be built almost every game too. Other items are based on your analysis of the game, so do it wisely:
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If you don't like Trinity Force or is dying a lot, very good item on Kha'Zix too, but I don't like the non-stack thing between these two items.
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Buy one or two of these if you like it. Can tell you that a Phage is much more cost effective as it gives you almost same AD, more health, slow and is built later.
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philosopher's stone Take this early game if you're having a very hard lane or can't get too many last hits. It's renegerations and gold generation are way too good on almost every champion.
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Very good item when laning against an AD, but like The Brutalizer, it'll delay your core build
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/ Good when laning against a magical damage dealer (most mid-lane, some Vlads or Rumbles etc. on top lane). Probably a Mercury's Treads should be enough for the laning phase, but take it if your death recap shows too much magical damage.
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Just amazing item, use it if any hard CC is shutting you down and you know how to use it properly. Think twice about not buying it against Malzahar, a Skarner, Warwick, Fiddlesticks, etc.
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Probably best defensive choice for most super squishies, and you're not exception. You can go in, kill one people, die, kill another people and made it out alive.
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If the enemy team is so AD centric and nobody on your team grabs it (should never happen), you can take it.
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Never bought it, but saw someone using it mid-game as a jungler, to increased health, MR and healing from Void Spike. Doesn't seem too bad, and it's a cheap item, I'll test it later and put some detailed info. However, max Void Spike first if you'll build it.
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Don't do it. Just put it here to say it's useless and don't buy it please.
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But it's not in the core? Yes, but buy another! Good if you're fed and not dying too much at all. You can even run 3 Bloodthirster on the build!
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Better magic resist item you'll find besides Quicksilver Sash. Buy it if their AP is wrecking you and your team.
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Very good early game, but you already are going to take Phage and Heart of Gold, and this will delay your core build maybe too much. If you plan to do this one, it can either be sold later or upgraded into a...
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It's good on Kha'Zix because of the ArPen and AD, but critical chance and attack speed isn't that great. You can use this if you find hard to burst people down and their team don't walk together often, AKA this active is a monster on small skirmishes.
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I saw some people building this... overall, not bad. This item is almost like Trinity Force, providing so much different stats that can fit most champions. However, this item is too expensive to afford, and you'll end like Jax with a "full-build" like Boots+TriForce+Gunblade.
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You can think this is better than Last Whisper, but as it took 3 AA to get it's full effective, I don't like it on assassin :( Also, it doesn't synergize well with Weapon Expertise , and if your target buys just a Guardian Angel, the %ArPen becomes better than Flat ArPen. If you like it or have good games with it, you can buy it, thanks for poisonedblood71 for let me know
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... As well as every AS and CrC (attack speed/critical chance) item, is not good on assassins. But if you're building bruiser (bruiser section coming soon (TM)) and not assassin, these can be options. I'll think about it later.
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Other items I'll update later, as it's just the release of Kha'Zix and not too much has been experienced. Leave a comment if you had a good game with another item!

"Where should I go with Kha'Zix?"
Most people will say top, others will say jungle. Yeah, he can do that pretty well, but as every assassin, he can go mid and probably do it better. This is a problems with assassin in the current meta. When they're AP (Kat, Kass, Ahri, etc.) you can just go mid and you're doing it right. But if you pick Fiora, Talon and Kha'Zix, they're too squishy to go top or jungle, even if Kha'Zix has the better sustainability among them and can jungle fine. This is why assassins are BAD PICKS on solo queues, draft or not. The exception for this issue is maybe Rengar, who is an assassin and can be very tanky, and is current a broken champion. However, Kha'Zix has more sustainability than Fiora and HAVE IT, unlike Talon; this made him usable on top and viable on jungle, and on premade with AP top/jungle, you can go mid.

Most people I see playing assassins, do it wrong. They just think about jumping in the enemy and killing their target. And they do it. But it's not what you're supposed to do. The assassin's job is to jump in the enemy, kill their target AND MADE IT OUT ALIVE!. If you died, you did it WRONG. But of course, if you initiate the fight by killing your target, you WILL die, you are among 4 man, there's no way to made it out alive. So, what to do as an assassin? Use the flank, come from unexpected places or even from nowhere, and more important, do it WHEN 1. your target is isolated (not only by your Taste Their Fear, really isolated) 2. the fight has already been initiated.
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If you don't find an easy target, by this I mean they're all grouped, go for the jungle and after the initiation, attack from behind or from the flank. As Kha'Zix, you have your Leap that do it easily. Other tactics include to stay with your team and use stealth or gap-closers to go for your target after initiation, and as Kha'Zix, you have Leap and Void Assault to do it.
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Another important topic is to punish your enemies for being out of position. The jungle need to be all warded when there's an assassin on the team, and if your support is not helping with it, you should do it. With this, you can see a carry alone in the jungle and make him a visit, for his regret. This will clearly give your team the upper hand in the incoming fight they "have to" initiate.

Starting with Void Spike will give you a better start against the buff camp, because of AoE damage and healing. For defensive jungle, you can start at blue buff for spammable Void Spike, then do entire jungle and if high on health (depends on rune setup) ready a gank. If not, just recall, buy a Ruby Crystal for your Heart of Gold and go for a gank. You're squishy, so keeping all time clearing camps will make you go low easily, always head for a gank when you see an opportunity, it's better for you and your team. For some more offensive route, just grab the red buff and put a point into Leap, head for a lane, leap in and slow them. Make sure to ping for your partner, as it can easily land a first blood if enough attention is paid. In any case, DO NOT use Flash to try the first blood, your enemy will probably use it if you've done it right, so just come back a minute or two later and he'll have no escape this time.
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Kha'Zix is not the better counter-jungling, as he's not that duelist, but he can leap walls and make a chase hard time, and after level 6 his juking is amazing. For deflecting, he's not buff dependant, but if you find your enemy jungler staring at you, you can't hold long enough for your teammates to come at hand. Of course, if he's a bad jungler and come to your jungle at low health, just burst him down for an easy kill.

DON'T LET THAT BASTARD TAKE YOUR HEAD!!! Instead, you should take his other eye as a proof of your evolution! Besides that "hidden passive" for a fourth evolution point that I mentioned before (that still a rumor), Kha'Zix and Rengar shares much more gameplay. Playing as Kha'Zix, YOU NEED TO MAKE THAT FREAKING BEAST REGRET FOR ENTERING THIS LEAGUE. Make sure the treatments when you play against a Rengar is like a relation between a kitten and a feral evolutionary Void monster, not a lion and a praying mantis. If you lose for a Rengar, you shouldn't be playing Kha'Zix at first! And if you're going to play Kha'Zix and some idiot on your team picks Rengar, DODGE THIS QUEUE! You're going to lose with this stupid kid on your team, trust me.
For those who don't get it yet, I'm just joking

Juking is outrageous. Juking is truly, truly outrageous. Juking is art in the form of war in the Fields of Justice. And Kha'Zix is the newest addition to the League of Juking, only unwelcomed by Kassadin for lore reasons.
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If you don't know what juking is, there's a good guide here on this website which can give you the headlines. But basically, juking is deceive your opponent to save your life and even bring a kill for your team.
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Kha'Zix can juke well with Leap and Void Assault, as you can see in my video below. Here are some tips:

• Leap has a very high range after evolved, making wall-leap easier and at longer ranges
• As Leap CAN be aimed (unlike Flash), you can see clearly if you're going to jump over the wall or fail
• Void Assault isn't a stealth system like Deceive or Thrill of the Hunt that will let you escape easily, because it's designed to proc your passive only, but don't forget that 40% MS is GREAT
• As soon as you enter a bush being chase, you can wait at the edge of the bush until you chaser come at you, then activate your ultimate as the chaser walks into the bush, letting you go for a direction without his knowledge. It's very effective at top and bottom lanes, where dwells the mighty double bushes
• If you Leap over a wall, and your only chaser Flash to get you, you can then Flash back to your starting point and then say in All Chat "im lebronk lulz noob"

Video recorded by me on PBE. Thanks to my brother (Trynda) for helping me with the video. Yes, I know I could have killed him, but I was just showing how to juke.

Well, this is only my thoughts on the newest champion Kha'Zix, and of course it's not the most accurate, accuracy of builds comes with time. However, I'm open to any opinion that can help me improve this guide except for those who think Phage is bad for a praying mantis
If you have any (good) experience with Kha'Zix using different masteries, runes and items, please let me know and I'll be glad to think about your case ;)
Thanks for reading, and keep killing those Rengars!
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Special thanks for JhoiJhoi and his/her incredible guide and for those who tested Kha'Zix on PBE with me :3

30/09/12: Revamped Evolution Points section. Thanks for all those who said something about it on the discussions. Added juking video on the juking section. Added more optional items, thanks to poisonedblood71 and HeikeR
29/09/12: Published, however, the icons are not all set yet :(
28/09/12: Improved situational items and changed Shen to Kha'Zix (I used Shen's icon to test formatting since Kha'Zix doesn't have icons yet). Just waiting Kha'Zix to be introduced in MobaFire to publish it
23/09/12: Wrote the guide, waiting Kha'Zix release on live servers to post it here