The Ultimate Predator ability comes from two factors being met:

1. There is a Rengar and Kha'Zix against one another.
2. Rengar MUST posses a Bonetooth Necklace and Kha'Zix must be Level 16. After these conditions are met, the side quest titled: The Hunt is On! will begin as Dalaren mentioned above. There are rewards when one or the other is killed after this quest appears.

This has been confirmed and released by Riot Games.

More information about the Rengar/ Kha'Zix side quest in my Kha'Zix Build here

Okay, I tested the penta kill theorey against bots. No ultimate predator. :(

I still don't trust you. I do a lot of photoshoping, and that is easily do-able. But, if it is true, I will get it. I will try to play kha' as much as I can to see if it's true. I will try to get a video on it :P.

What I mean about Youmuu's Ghostblade, is that you should put it as like the last build item on the cheat sheet, or sell brutalizer for something different. It makes it "look" more complete. There are obviously more important items than youmuu's, as he doesn't really need attack speed. Put it after guardian angel, or sell brut and buy something else.

I wonder if ultimate predator works on tt. Hmmm... Also wonder if it works against bots, would be soo easy that way.

I mostly play kha'Zix as a jungler, so I build him slightly differently. It would be nice if you had a jungle build too. :D

If you had some videos, we would all know how awesome you are kha'zix... It would be cool to see your match history with your builds too.

It's nice to see someone trying to make their guide great, make it more in-depth, and it may be in the top 3. Good Luck! :)

To JollyCheong: Well, Hextech Gunblade is nice, because it have everything you can use as Kha'Zix. AD, AP for passive and heal from Void Spike, Life Steal and mainly Spell Vamp, which will heal so much, because you are focused on damage from spells. But Hextech is just possibility and you take better items most time.

To Doom Lord: You can always build Youmuu's Ghostblade from The Brutalizer, which is core item, but you will have to focus on more important items such as Trinity Force or Bloodthirster so you will haven't got enough money (Yomuu's is good, but don't worth it at the time).
Btw. Why you don't trust me about Ultimate Predator? :D I've seen it once and I only helped our Kha'Zix. But as proof: there is link to official WIKI: Quest is called The Hunt is On! and link to thread on LoL forum with screenshot of Rengar's trophy here, which is more provable. And about that pentakill, I am not so sure now, I've just read it somewhere, so try it and let me know :D

You really should put youmu's in the build at the top, even though you say it later in the core items.
I would also get the vampirism instead of havoc, even though you get the scepter, but that's just personal choice.

I never heard of "ultimate predator". I've never gotten a penta, just a quadra ;) as Kha'Zix, and I've fought a few Rengar's as him too. Perhaps the games never lasted long enough... I'm assuming this is a troll part of the guide :P.

All in all, it's a decent guide, but I feel that it could be better.

Hextech gunblade should not be an option.