I've been hearing a lot about this, but despite having played Kha'Zix against many a Rengar in 3's, I have not once seen the little buff appear and certainly never had the event finish. Is it exclusive to 5v5?

What buff and event may you be referring to?

there are some boundaries for this event, both kha'zix and rengar have to be level 16, kha'zix needs to spend his 3 evo points, and rengar needs his necklace

All those boundaries have been met for me and i have also not had the pleasure to have the hunt go on. Especially since I tend to be scoring aces at that point in the game and a 4th evo point would be very well spent.

Exactly, I seem to meet all of the requirements as well but once again it appears that Twisted Treeline is lacking... I heard that Rengar has to reveal you during your stealth with his ult. I have not tried that yet, but it might be the key?

That would be a pretty epic trigger for the event, regardless of if it is the case. It would explain why riot's official stance is that it will happen "at some time after the conditions are met". RNG have never been riot's thing.

http://leagueoflegends.wikia.com/wik...ix/SkinsTrivia

One of them needs to kill the other after the requisites are met for the quest to POp, when either Kha'zix or rengar kills the other (a straight kill, not assist) you'll get the quest popping intead of te reward, you'll need to kill the other again to get the reward.

It doesn't work in Ranked games.

I have had it on summoner's rift, Kha'Zix's head as Rengar, however I only got the notification after I had gotten 14 trophies on my necklace, so I think you need max trophies on your necklace, hope that helped.