At long last, I finally have it!!! I beat Atheon on Hard for the 6th time tonight (thanks largely to my awesome Raid team; also beat Crota on Hard for the very first time) and it finally dropped for me! I can't believe it! That leaves Necrochasm as the only Exotic I've yet to unlock. I have Eidolon Ally, and thanks to my stellar Raid team, I also have the "Glowhoo" shader; which I'm told has to have been unlocked at least once before the Crux of Crota will drop. That's for some other time, though; I only just started upgraded my new Vex Mythoclast, and I wanna get this baby maxed out. I still think they should improve the drop rates in Vault of Glass to be more like Crota's End; from what I've heard on these forums, I'm think I got REALLY lucky getting the drop on only my 6th attempt.