In Destiny, the new exotic weapon, the Necrochasm, is currently not in the game.
While Crota kills in hard mode of Destiny are still quite rare, there have been a few. However, there hasn’t been a confirmed drop of the “Crux of Crota” yet. This item is supposed to allow crafting the Necrochasm from the Shroud of the Pit/the Eidolon Ally: An exotic auto rifle that fires incredibly fast.
At the moment, it seems that this drop is either not possible at all, really, really rare, or that certain conditions are tied to the drop.
The most likely option is probably “The drop is not possible yet”.
The “Husk of the Pit” (Shroud of the Pit) was only unlocked with another patch that came out of the ordinary. Perhaps it will be the same with the Necrochasm, that it will be added to the game later.
Currently, as is often the case, there are already some rumors circulating. Some suspect that you have to defeat hard mode Crota in Destiny while using the fully upgraded Eidolon Ally or at least having it in your inventory.
Update 23.1. 2:20: A somewhat nebulous update. Bungie has officially commented on the Crux of Crota. Community Manager DeeJ has asked the developers. And received confirmation: The Crux of Crota is in the loot table. And will drop. He is following up to confirm. As of now, not a single drop of the Crux of Crota and thus the Necrochasm has been confirmed.
