Hello, my Name is Bats293, ingame known as un1t3dg4m3r, I am a Gold player on EUW, and wasted ~1.5k hours playing this game. ;D just kiddin
This is my first Guide, i hope i can teach someone how to play this champ, have fun reading!
Here you can see/prove my statements
And here's the 2nd
I am maining jungle since the start of season 4, and prefer assasins, due to their snowball/carry potential.

Pros +Nice DMG +great Mobility due to Leap's cooldownreset + Void Assaultgives huge passive buff to ultimate + great sustain due to Void Spike + one of the best counterjungler due to the passive of Taste Their Fear

Cons -high risk high reward champion -doesn't scale off that good like Shyvana or any other late game jungler -no stun, only 2 slows - extreamly voulnerable to any Crowd control

PASSIVE

Kha'Zix's passive gives him a buff that causes his next auto attack to deal additional damage and slow. He gains the buff whenever he can't be seen by anyone on the enemy team.
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Q

Kha'Zix's Q is a single target, melee range spell that deals more damage if the target is isolated. It has a low cooldown so this is Kha'Zix main ability that he uses when fighting in melee range.

Evolving Kha'Zix's Q increases the range of his Q and auto attack by 50 and makes it deal a lot more damage to isolated targets.
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I would evolve this ability on the 3rd place, it's a huge damage amplification, but the other 2 spells are far more usefull then this
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It has a CD of 3.5 sec
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W

Kha'Zix's W is a missile that deals AoE damage on impact and slows the targets that it hits. If Kha'Zix is standing next to the impact, he gets an AP scaling heal. This ability does extra damage to monsters.

Evolving Kha'Zix's W causes him to shoot three missiles instead of one and it increases the slow along with granting vision of the target for a couple seconds.
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Evolving this 1st is pretty good, due to amplifying the slow, and amplifying the possible spot it hits.
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It has a CD of 8 sec
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E

Leap is kha'zix main mobility spell,it has pretty high cooldown which makes it difficult to chose whether to use it to engage, or to follow up.
Evolving it increases the rage dramaticly and gives a reset on kill/assist
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Evolving this ability is essential, whether on lvl 6, or lvl 11, you need it, asap
Kha'Zix is jumping and getting out squishy tragets. That CD reset is important
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It has a CD of 22 sec which lowers by 2 every lvl
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R

Kha'Zix's R is a short duration stealth with 2 charges that increases his movespeed while stealthed. This ability in itself isn't that strong, but it also lets you evolve one of your other abilities every time you rank up his ultimate. One bonus with this ability is that it resets Kha'Zix's passive when you use it so your next auto attack will deal bonus damage and slow.
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Evolving this has been nerfed pretty hard, not viable at all now
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It has a CD of 100 sec which lowers by 10 every lvl

Why do I chose to pick:
Greater Mark of Attack Damage x9
Greater Seal of Armor x9
Greater Quintessence of Armor Penetration x9
Greater Glyph of Magic Resist x9

How always, start blue, get red, gank mid, then farm your way up to six. After you have w evolved, your ganks are way stronger

That is a pretty trick one in my opinion. In Teamfights you have to get the carries, AP or ADC, focus the squishys first, due to your reset

I hope you've liked my first Guide, upvote if you did!
Help me improving by writing a comment.
See you Guys on the rift!