At the very beginning i will say i'm sorry mines not super fancy and have all of those fancy icons and stuff I'm still figuring out the coding for it all but here goes nothing ^_^
Welcome to my all time favorite champion Rengar build. Okay when you first look look at the build you may think what the heck is with these weird items such as the Ionic Spark. But at least try it it actually works i kick trash with this build. So bear with me i will explain the reasoning behind items, masteries, and runes so lets dive in.

Pros

• very good assassin
• Amazing early game
• Good farming capabilities
• skill that collaborate well
• good at escaping

Cons

• squishy early game
• not very good without farm
• CC=Worst enemy
• weak against tanky Champions.

So lets discuss the runes shall we?
Okay it is going to be like this here you go. You pick up 9 greater mark of desolation I will give the reasoning behind the armor pen. runes at the end of the rune section.
For seals we have the basic 9 Greater Seal of Armor for some flat armor no real questions to it.
Again we have basic bruiser seals (shown above) and the basic bruiser glyphs so we are going to pick up 9 Greater Glyph of Scaling Magic Resist.
So we are going to get 3 greater quintessence of desolation. This gives us some much needed armor pen.
Ok so the reason we are going to get straight armor pen. runes for quints and marks is because i have noticed you don't need the damage from runes. With Rengar the damage isn't necessarily needed from the runes because when someone builds armor against rengar you are screwed. This armor pen. is in the end very very much needed.

Boots of speed will be the first item we pick up along with three health pots. These are for the early game aggression so if you get hurt or need to have a little extra movement to catch up to the enemy.

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bonetooth necklace The bonetooth necklace will be the next item to pick up. So we will be picking this up as it is Rengar's signature item. But not only that it is the ultimate snowballing machine which is amazing late game.

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The heart of gold will be the next major item as it will build into the Locket of the Iron Solari. One of the biggest reasons i am picking this up early game is for the GP10 and health for survivability.

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So now we are going to use those early game boots and build them into the Mercury's Treads! the reason i have chosen these boots is mainly because of the tenacity, because since CC is rengar's worst nightmare this tenacity helps a lot. The magic resist is very helpful too.

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OR So next up is what may seem like a very weird item on rengar but believe me it helps. So how the Ionic Spark helps is the attack speed and passive combined with Rengar's Q is amazing damage. And what makes it all better is it also gives health for even more survivability! But if this item doesn't click with your play style i will have some replacements in the next section. But in the second build you can pick up last whisper in place of Ionic spark for damage and armor pen. if you prefer over it

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so this next big item is just flat out amazing on Rengar. It comes with damage,health, and an on hit slow! In my opinion this item hits almost every category Rengar needs.

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we now move on to Bloodthirster this item has got life steal damage and scaling damage per CS gained while in possession its just insane AD for Rengar late game it's still pretty good because he get better at CS and is just amazing altogether.

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This is the end of the build for it we are going to build the heart of gold into it and what's great about this item is the defense, health, and damage reduction it provides. it may seem stupid to some but it just works one way or another.

First off with the skills we are going to discuss what exactly each skill does starting off with your Q Savagery.So what Savagery does is that it is a next hit skill. So when you activate it the next attack you do deals extra damage and gives an attack speed bonus. Now what I have to add to that is the ferocity bonus from his passive Unseen Predator. The effect is is that Rengar deals 170% damage of his attack damage and he gets and even greater attack speed bonus. What you want to note is that that can stack if you cast his normal and then ferocity bonused Q and it gives him a ton of attack speed. What i have noticed is that most people don't realize his Q works on turrets too which makes him an amazingly excellent pusher.
His next skill is his W Battle Roar. So what Battle roar does is he lets out a roar and it deals magic damage to surrounding enemies and grants him extra armor and magic resist for a short period of time. With the ferocity bonus it heals 15% of his total health which can be a saving grace sometimes. You can also use this when you are in a team fight you can activate it at the very beginning for the armor and MR bonus with is very helpful in the end.
So now is his E Bola Strike. His E is a small bola that he throws that damages and slows a target enemy within a small range. The ferocity bonus is that it roots the target for a very short duration and then slows them.
Finally we have his ultimate. This is his R Thrill of the Hunt. This is Rengar's ultimate what it does is it provides a 7 sec stealth that builds up ferocity fast and shows you every enemie within a set distance depending on its rank.

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Ok so now we are going to go onto Rengar skill uses and order. So we are going to pick up Savagery mainly because it is Rengar's main damage ability and helps if you are aggressive and go for first blood or even for farming for you more passive folk. You will max this ability first obviously. Next will be Bola Strike because if you have a good jungler and he ganks early you will use this to help in those ganks. In the end your E is going to be maxed last because it is helpful in ganks and team fights but it doesn't help as much in the top aspect as the extra survivability attainable with Battle Roar. This is your ability you take at level 3 and then max 2nd. Again as i explained it offers the extra survivability because at max ferocity it heals you and even when not healing it offers extra armor and magic resist. And of course his R Thrill of the Hunt you wanna take this whenever possible at levels 6,11,and 16. That is it for your Skills we will now move onto Summoner Spells and then finish up with a summary.

So what i suggest is taking Ignite and Flash. Ok so personally i prefer flash over ghost but if you want to take Ghost in place of flash as a replacement if you like that more. And if you want to get rid of ignite because you just don't like i guess i would suggest replacing it with Exhaust or Heal for the sake that they are the only possibly good summoner spells i have used on Rengar or at least found useful.

So that is it and thanks for reading my guide i hope it works for you as well as it worked for me. If you have any suggestion or critiques please post them i am very open to suggestions and would love the help. That is it for now i might add more to my guide later on but this is it for now thank you if you made it this far. I say thank you to JhoiJhoi for he guide on how to make a guide which you can find here http://www.mobafire.com/league-of-legends/build/making-a-guide-101506 And always remember this

He is always watching you!