Hello, and welcome to my Rengar build. I will cover the basics of what this build is and why I use it, along with some tips I've gained over time.

Here is my OP.GG page if you're into this stuff.

I'M NOT GOING TO PUT IN THE TIME FOR FANCY CODING AND STUFF. THIS IS A QUICK, INFORMATIONAL GUIDE.

Rengar can be played many different ways, but I enjoy playing him most as an assassin. With this build, your only job is to delete the squishies on the enemy team, i.e. Lux, Jinx, Master Yi. Therefore, I do not build a ton of armor penetration. This build is optimized to reduce the amount of backliners they have, NOT to fight the tanks. Leave that to Vayne. Because this is an assassin build, we don't normally take defensive items at all.

Pros:

With a little lead, can delete enemies with ease
Can splitpush well and escape
Has crazy lane dominance
Unstoppable if ahead

Cons:

Has many counters
Squishy AF with this build
Not so great if behind
Is not a walking CC tank (looking at you Nautilus

Flash and Ignite are what I like to take. Ignite for lane pressure, and flash to make up for mistakes we all make, or to finish up a kill. Teleport is ok too, but since the CD is so long and you can run around pretty fast, I don't think it's worth it. Exhaust is alright too, but I think ignite is better most of the time.

I take flat AD reds for last hitting and since most squishies don't build much armor, and we have plenty of armor pen in our build. Flat armor yellows for less damage from minions and AA's early. Scaling MR blues since most (MOST) AP champs don't do huge amounts of magic damage till late game. Flat AD quints, same reasons as reds.

You can take other runes too (CDR, AS, flat MR, armor pen) but with this build I don't feel that they are necessary.

Masteries, go 12/18. Make sure you have Assassin , Dangerous Game , and Thunderlord's Decree .

GOOD ITEMS

Tiamat is core. It is HP regen, AOE waveclear, AA reset, damage, etc...Only upgrade to Ravenous Hydra very late game.

Youmuu's Ghostblade is great for its active, dmg and armor pen. Makes roams easier. Also helps taking towers.

Boots of Swiftness are good for roaming. I like these more than the CDR boots since we'll hit max CDR with other items.

Last Whisper. Buy this before Triforce if their squishies are blue build ezreal (iceborne armor), Graves (passive armor), or some AP champ with Zhonya's Hourglass. Don't finish into Lord Dominik's Regards until late game. If you need lifesteal, finish Hydra first. If you need more armor pen, buy this first (shouldn't need more armor pen than this often, though)

Trinity Force offers many stats in one item, though it is expensive. Build Sheen first for dmg, Phage for chase, some HP.

Essence Reaver is a good item. Get after Triforce. It gives dmg, CDR and crit chance.

BAD/OK ITEMS

Black Cleaver is good against tanky tanks. I would not recommend it over the above item because you are trying to delete squishies, not fight tanks.

Mercurial Scimitar is good if you're getting cc'd. Buuuuut, you should not be getting close until they have wasted their spells.

Infinity Edge is a replacement for essence reaver. I like the CDR from ER more than the bonus crit damage.

Rapid Firecannon, Statikk Shiv, and Phantom Dancer are all alright. The crit chance, movespeed and burst are nice, but we don't really need attack speed because we have Q to reset AA's over and over.

Blade of the Ruined King is a decent item. It doesn't offer a whole lot of damage and no CDR, but it's good for dueling HP stacking champs, which you shouldn't be doing anyway.

Buy your starting items and walk to lane. When the enemy minions walk up, jump on them and get them to hit you a little. This makes their minions push faster, which means you won't be so far up in lane. Your levels 1-5 are really strong. Against tanky champs that you can't kill, just try not to push the wave too fast. Hit lvl 6, farm hard then roam mid/botlane. Against squishier toplaners, if you managed to kill them 1-2 times early, you should be able to push your wave and if the jungler ganks, you probably can get at least 1 kill (i've gotten lots of doubles when the jungler ganks).

Mid game, your job is to roam. See an ADC farming a lane by themselves? Press R and delete them.

Late game, when there are non stop teamfights, you have to play really far back. Like out of range of anything. Don't go in until important abilities are used yet; silences/knockups/knockbacks/stuns really screw you over. Go for the squishiest high damage dealer, so the ADC most of the time. If they suck, go for their midlaner or squishy jungler. Once the squishies are dead kill whoever you want.

TIPS

Bushes are your friend. Abuse them.

You can flash into a bush to make enemies lose vision, then jump on them from there.

In teamfights, play really far back. Be the douche who only goes in to clean up.

Once you get 6 bonetooth necklace stacks and some movespeed you can jump over baron/drag wall.

You can juke people really hard when they're chasing you and you ult. Then walk back the other way. Throws a lot of people off.

Don't ult unless you have 4-5 stacks of ferocity, unless you're just trying to escape.

COMBOS:

tl;dr Once you hit 6, press r then q then e then w then mash q and AA

AA reset burst in lane (can be lvl 1):
(start in bush) AA Q AA

lvl 3-5 burst:
(start in bush) AA (while in midair) E W (once you land) Q AA AA

lvl 6+ all in if they can't outrun you:
(with 5 ferocity) R (Q while invisible) (E W while in midair) Q AA Q AA AA

lvl 6+ all in if they can get away, but are squishy:
(with 5 ferocity) R AA (E W while in midair) Q E AA Q

full, full combo with items:
(with 5 ferocity) R youmuu's Q (E W tiamat while in midair) Q AA Q

Thanks for reading my guide. If you liked it, upvote plz! :D If not, leave a comment and I'll take it into account.