Hello,
I decided to conduct a guide of Rengar in jungle, a champion i love playing and never get bored.
This build/runes/masteries etc. are coming from my personal experience and testing.
If you have any suggestions or objections with this guide please let me know by commenting on this guide.

Enjoy the anger of rengar

Pros of jungle rengar

Manaless
Nice sustain with Ferocious Roar
Nice damage with Q+Q combo
Sick ganks providing both damage and CC
Sicker ganks after level 6
Easy snowball ability
Easy backdooring
Has unique item
Fun to play
OP skin


Cons of jungle rengar

Needs 2 or more ferocity to gank properly pre-6
Needs early kills to snowball hard
Only one skin

Marks: Attack damage x 9: Cheap, strong early game and good early damage. The need of attack speed doesnt exist because of the Quintessences and the rengar Q

Seals Armor x 9: Cheap, for tank purposes and early game survivability from enpowered creeps.

Glyphs Magic Resist x 9: Cheap, adds some much needed magic resist which lacks from the build and gives early game tankiness

Quintessences Attack speed x 3: A must for junglers whose damage comes from autoattacks. Those runes enhance your early game damage and with the attack speed buff from Q+Q combo makes rengar crazy (with some life steal they do wonders)

While in brush or stealth, Rengar will leap at the target when using his basic attack. Rengar builds 1 Ferocity with each ability he uses on enemies. When reaching 5 Ferocity, Rengar's next ability becomes empowered, granting it a bonus effect.

This passive has 2 parts.

1) The first part is rengar's jump from bush or stealth. The jump has 600 range normally and 750 range if you own a nine-stacked bonetooth necklace. You can use the jump for around 0.5 seconds after leaving the brush and can be used repeatevely. This jump is rengar's gap closer which can be very effective during ganks or when chasing someone in the jungle. Because of his jump you should prefer teamfighting in jungle where you can almost always catch a fleeing enemy.

2) The second part refers to the ferocity bar. After using 5 abilities which hit an enemy, Rengar will reach 5 ferocity and all of his enpowered abilities will be ready to use. Note that normal abilities' cooldowns wont be reset.

Rengar's next basic attack deals bonus damage and grants him increased attack speed for 4 seconds.

Savagery deals bonus damage equal to 150% of Rengar's attack damage, and the Attack Speed bonus is doubled.

This skill is rengar's main damage output. Savagery gives nice attack speed buff helping your clearing time a lot. It resets his attack timer so using it right after an auto is recommended. Combined with Empowered Savagery it deals both of burst and sustained damage which helps taking down important targets (enemy ADC, APC) and helps contributing in ganks.

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Rengar lets out a battle roar, damaging enemies and gaining bonus armor and magic resist for 3 seconds.

Battle Roar also heals Rengar for 40 (+20*level).

This skill does little damage in fact but it provides a nice armor and magic resist buff for 3 seconds making you both tanky in teamfights and sustained in jungle. Empowered Battle Roar also heals rengar providing another source of sustain in jungle and reduces the need of pots.