As part of the event, The Hunt Is On!, 
Rengar and 
Kha'Zix are assigned to defeat one another in battle. The first to score a kill on the other claims victory (assists count). The event begins randomly after Kha'Zix has used all 3 evolution points, and Rengar is in possession of the Bonetooth Necklace Bonetooth Necklace.
- If
Kha'Zix is victorious, he gains a fourth evolution point. - If Rengar is victorious, his Bonetooth Necklace Bonetooth Necklace is replaced with the Head of Kha'Zix Head of Kha'Zix, which grants him the bonuses of a fully stacked Bonetooth Necklace Bonetooth Necklace.
This event can only occur once per game, even if both teams possess both champions. A video describing the event can be found here.