Hi! This is my first guide, so do not throw shoes in me, if I forgot something or are not clearly explained.
My older brother bought this champion and was disappointed. Difficult, fragile, with low damage and mana, he seemed completely useless for my brother. I gave him advices, such as: buy Brutalizer and Bloodthrister as Riot recommended, and you'll be ok. But when we played, he still feed and lost his line. Then I decided to try Kha'zix myself. I was surprised, but my advices really didn't work. I tried other builds I found online, but they still wasn't good. Champion is too fragile, and didn't do too much - just couple hits, and then died. After some days I made this build, and it works. I owned the battlefield.

You should know, Kha'Zix isn't OP (overpowered) like many people think. He is very vulnerable to croud control (stuns, fears and many other) and not the easiest one champion to play. If you are playing right, you can be beast in 3 vs 3, but in 5 vs 5 you still must be carefull.

Masteries is obvious - max your damage and spend rest of points to protection. If you go to jungle get Butcher and Tough Skin. But most game you will be ganking, so you can jungle even with this.

There is some difference between builds:

Assassin build The Brutalizer.

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Offtank build

Runes is obvious too - you need damage and some defense.

But Assassin build is must be about cooldown:
And Offtank build is more about armor:

IT'S UNDER CONSTRUCTION - COMING SOON

This is the most important part of build. Get Boots and Bottles for laning. Two Dorans for beginning of the game. They will give hp, damage and some lifesteal for standing on lane longer. In combine with your Void Spike it helps you really well. Then you go for Phage - more damage, more hp and nice slowing proc well synerging with you. And then we come to core item - Trinity! This gives all you need: AP, AD, AS, crit, hp, mana, speed and two procs. There is no any other item compared to Trinity.
Let's do little brake and look why Trinity, not Frozen Mallet. FM doesn't give u AP, AS, crit, mana and damage proc, so... any more questions?

Upgrade your boots, usually (and against AP) it will be Mercury but if you are sure there is not too much CC and you're well fed, then you can do Ionian. Against AD you need Ninja Tabi, but not too often.

Then we go for defense items. HP, armor and Hexdrinker, which one will give you damage and survivability against AP. Warmog? Hmm... am I tank or something? Sure no, but with this one you can forget about your fears to be r**ed in teambattle. Remember that you are assasin, and you will be the target number one if something goes wrong (95% of any game). And this is the best item against true damage Darius ulti.

Finish armor into Atma and Hex into Maw. Now you are ready to cut most enemies 1 vs 1. If game still doesn't end then get Whisper (with this item even overfed Darius runs like girl away from you).

Now discuss about other popular items for him. Youmuu. Actually it sound like "you moo". If you have couple horns maybe you need it. Ok ok ok. Lets look on our other items - all of them is about survivability. Youmuu is not. You can get it if your team want you pushing active/slowing enemy team, and you evolved Void Spike, but most times you shouldn't.
Bloodthrister. You don't need lifesteal, because Warmog and Spikes will heal you enough. You are squishy and can easy lost most of charges. And it's very long farm for B.F.Sword - time, which you can spend with more effective items. You may get it only when you are overfed.

There is everething clear, like in runes/masteries. Taste Their Fear - your main skill, so max it first. Then Void Spike,last one Leap and ulti when it's available.

Evolve first Leap, because you really need it - without it you are just slow bug. You can't kill anyone if you can't reach him and you won't run away from your enemies if you're jumping like pregnat elephant.
Then evolve Taste Their Fear - your main damage skill, but remeber - evolve bonus works only on ISOLATED targets.
Last one is Void Assault. More invis = more confused enemy = more possibilities to run away = more passive slow. 40% damage reducing is very nice too, when troubles catch you (95% of battles).

When evolve Spikes? As I told before - it's very team depending. If feel you really need it, and your team needs, then do.

As it's guide under building, I'll try to put most detail here.

Gaming:
Go on top.
Harass a lot - Q, attack, go to bushes and back, Q and attack again, repeat it one more time, Ignite, FB.
Gank a lot, Trinity procs will help you. Use your ulti in battle to get passive and slow target again or run away with it. 1 sec gives you time to think what to do next. Be careful - if don't use all charges of ulti, soon it will go on cooldown anyway.
Remember that you are squishy.

Farming: with nice AD items it won't be hard.

16/10/2012 Added one more build, alt+tab guide, and main 'offtank' build become alternative.
08/10/2012 Guide created. I just forget when it happens.

Hope you guys like this guide and rate. Please tell me what else you want to know about Kha'Zix and I will add it.

Don't forget to comment!