As covered in this Destiny guide. Destiny power leveling also has guides for you, the Necrochasm is obtained by getting the Husk of the Pit from a Swordbearer Knight and using an Embalming Orb to upgrade it to Eidolon Ally. The final evolution to Necrochasm requires the Crux of Crota, which can only be obtained by beating the boss of Crota’s End on Hard Mode.
Millions of Destiny players have defeated Crota’s End on Hard Mode since it was released last Wednesday, according to Bungie.TheCrux of Crota has not dropped, however. It appears that it will require multiple completions Crota’s End Hard Mode to obtain.
Reddit user JoKeR2092 spotted a German player who has gotten the Crux of Crota to drop after multiple tries. So far, the Glowhoo shader appears to be a guaranteed drop the first time you beat Crota. The chance the Crux to drop appears to be amazingly small. Destiny player oSkyllao completed Crota’s End four times before obtaining the Crux of Crota. Many other players have completed the raid more than twice. So, it may take completing the Raid several times before you see the Crux. You can check out gameplay of him using the Necrochasm in the video below (Note: Everything is in German).
The good news is that when the Crux does drop and you upgrade the Eidolon Ally to Necrochasm, there ares no additional upgrades to perform. The Necrochasm comes maxed out as an Exotic Auto Rifle with all the perks finished and no additional work to do. It also appears to have a new 339 attack level, versus the normal 331 attack level for all other Exotic and Legendary weapons since The Dark Below was released. It’s still not clear on if the low drop rate for Crux of Crota is intentional or a bug with Destiny. Community Manager David “Deej” Dague confirmed recently on Reddit that it is part of the Crota’s End loot table, but would not give a hint as to its drop chances or to any possible prerequisites.