At around 45 min a 5v5 a quest appeared that said Zah'Zix must kill Rengar to be the ultimate Hunter. This was with a Zah'Zix on our team and a rengar and the enemy team. This is apparently an event. Anyone who has any extra info on this please post it here.

If you read their lore, it states that they are the 2 greatest hunters in the land who are only each others equals. So it's basically a roleplaying thing and (from my experience) if Rengar kills Kah'zix his trophy item's icon turns into Kah'zix's head. I'm not sure what happens when Kah'zix kills Rengar, but its probably something similar.

I've heard about the Bonetooth Necklace that Rengar gets turns into Kah'Zix's head. Also, I have heard that if Kah'Zix kills Rengar, Kah'Zix gets to evolve a fourth time instead of the normal three times while leveling.

This sounds awesome. I want to put this to the test.

http://www.youtube.com/watch?v=1PzLWxnvqgY

Watch this
there are some requirements
Its a a secret interaction

Yah just got this secondary quest its pretty cool.

When *Kha-Zix kills Rengar, he gets an extra evolution point, which gives his abilities extra (awesome) options. It makes his kit a little more potent.