The holy grail of weapons in Crota’s End is the Necrochasm. It is basically the equivalent of the Vex Mythoclast. But instead of this new exotic just dropping in the raid, it is instead the “final form” of the evolution of Husk of the Pit.
Weapon evolution is a new feature in The Dark Below where you take a weapon, kill a bunch of Hive, and use either Embalming Orb/Runed Core (acquired from Eris Morn Rank 3). This will take, say a rare gun and evolve it into a legendary gun.
Husk of the Pit drops randomly from the enemies called “Sword of Crota”. The problem with Husk of the Pit is that it is very unassuming and looks gray like a common item so you might dismantle it without even thinking about it:
The evolution tree of Husk of the Pit:
So don’t dismantle that “crappy” level 10 gun!