Rengar is a melee assassin champion, who is an extremely well jungler, but can also lane as well. He fights using high damage abilities to take his enemies down quickly. In this guide, I will explain his item build, masteries, runes and tricks he can use.
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[img=21/9/0 in masteries taking all the pure damage and armor penetration, as well as some extra damage to minions and monsters for jungling and farming. In the defense tree, i take the magic resist and health masteries, for extra durability.

In runes i take pure damage marks, pure armor seals, magic resist per level glyphs, and pure damage quintessences. This helps with his abilities at the beginning of the game. The marks and quintessences help with more damage that can get kills in the beginning. The glyphs and seals give extra durability throughout the game against mages and fighters alike.

Start the game with either a Doran's Blade or Boots and 3 Health Potion.
Build to Mercury Treads or Ninja Tabi as needed for durability which Rengar needs to survive team fights or just fights in general. The second item to get is the Bonetooth Necklace, Rengar's own item which i find extremely useful and just flatout badass. Next pick up a Pickaxe and build for a Last Whisper for extra damage and extremely high armor penetration, which will do extreme damage against almost everyone this early in the game. Now time for durability. I like to build for Guardian Angel starting with Chain Vest against AD teams or Null-Magic Mantle against mages. Next buy a B.F. Sword to build for Bloodthirster for extra damage and attack speed, which Rengar gets extremely powerful at this point. Then buy another B.F. Sword and build for your last item, Infinity Edge. Either of the last two items can be replaced with A Frozen Mallet.

All of these items can replace the Bonetooth Necklace if you don't want to snowball.
Force of Nature is a good choice, due to his low mobility, and not really any way to gain health but Heal or recalling.
Phantom Dancer is another good choice because of the attack speed and more movement.
A Frozen Mallet is a good choice for Rengar due to the slow every hit, and extra health, which makes him even more durable.
Bloodthirster is a good choice for any AD champion, even Rengar can use it well and wipe out the jungle faster with the life steal.
These items can replace the bonetooth necklace if you don't want to get it.

Heal is a good choice if Rengar is in a tough fight.
Smite plain jungling spell.
Flash since he has no other escape tool.
Ignite for that extra damage that can pick up the kill.

Pros

• extremely high damage
• great ganking ability
• fighting in the brush is an almost certain kill

Cons

• pretty difficult to get used to
• not very durable
• low escaping power

Savagery: Makes his next ability do extra damage(100% of your max AD)
Battle Roar: Does magic damage in an area.
Bola Strike: Does damage and slows an enemy.
Thrill of the Hunt: Stealth's for a few seconds, increasing movement speed and rapidly produce ferocity points, also reveals all enemies in an area.
Ferocity Bonus
Savagery: Does EXTREMELY high damage(150% of your maximum attack damage.)
Battle Roar: Increases the range and damage.
Bola Strike: Deals extra damage and stuns the enemy.
Offensive, tanky, and jungling you should get: If you are building Rengar AP, you should get:

I start at the wraith's Camp, followed by the wolf camp. Next I move to the Lizard Buff, followed by the Golem camp. Lastly is the Ancient Golem buff. The entire time look for ganks. At level 11, Rengar can kill the dragon if used correctly.

Ferocity is gained by using 5 abilities, or Thrill of the Hunt. Farm minions with abilities to gain ferocity, use a Furocious Savagery on the next champion, followed by Thrill of the Hunt to gain more, then use it again. It does so much damage they should die with just those two abilities, if not, use Bola Strike to low if not kill them leading into a kill with regular attacks(if they SOMEHOW haven't died yet.)

Mercury Treads do good against mages.
Ninja Tabi against AD champions.
Ionian Boots of Lucidity for faster ability use and faster ferocity boosts.
Boots of swiftiness for escaping better and chasing, since Rengar doesn't have very high mobility.

When you fight Rengar, stay away from the brush. Try to lure him out or just use an ability in the brush he ran in. A Sight ward is your best friend against Rengar, you know exactly where not to go. If he starts to go invisible... RUN!! He is about to do extreme damage and a turret is your best friend now. Again if he starts using abilities on minions, push him back and stop him from getting that ferocity. So if he can' use the brush, can't get ferocity, then he has to fight you one on one, now don't underestimate his dueling ability either though, he still does a lot of damage.

Rengar relies on chasing his enemies and doing extreme damage, in order to survive most of the team fights that you are in, you are going to need some good durability, as well as survivability. Building Maw of Malmortius and a Frozen Mallet, Warmog's Armor, Atma's Impaler combo, he will have plenty as well as killing ability.

Battle Roar can be a very powerful ability and can keep Rengar alive in many encounters. With the ferocity boost, he gains massive health, and since this move already does 100% ability power added you will be doing a lot of damage. I also tried to keep Rengar powerful AD too with Hextech Gunblade.