Well, I have seen two people with the necrochasm already. One, is Veroli, which is a 29 titan. People have already said they are a dev but no one is sure.
The other, is a 32 warlock. http://www.bungie.net/en/Legend/2/4611686018439791253/2305843009245506237#gear
He may not have it equipped so here is the Destiny Tracker link: http://destinytracker.com/destiny/overview/ps/Oskyllao
Yes, he has necrochasm. How he obtained it I don't know. It could have been using the ediolon ally or the embalming orb theory. But at least we know that it is a drop. Extremely rare, as it seems.
P.S. Sorry if the post is a bit sloppy, first time posting on a forum. :)
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You do know that Crota can drop the Crux of Crota which is used to upgrade the eidolin ally (no clue how to spell that lol) to the nechrochasm right?