Everyone in this thread is wrong. This is how it works. Upon kha zix and rengar both hittimg level 16 and being alive, and rengar has a bonetooth necklass (it doesnt need trophies) a secondary quest will appear for EVERYONE. If rengar kills or gets an assist for killing kahzix, he gets the head of khazix, which is the same thing with a little buff, and it doesnt lose trophies. If khazix kills rengar or gets an assist killing him, he gets a fourth evolution.
They are starting secondary quests for summonners rift based on lore. I think they are going to do one for kayle and morgana, nasus and renekton. And so on