The Necrochasm questline involves many steps to get the exotic auto rifle. This weapon will require you to hunt down Hive enemies and wait for the Husk of the Pit to drop. From there, you must farm various hive enemies to upgrade the Husk of the Pit into the Eidolon Ally rifle. Once you have this, you must complete the 390 version of Crota’s End, and defeat Omnigul in “The Will of Crota” strike. Once this is complete, you return to Eris to get the “Essence of Oversoul” to infuse into your Eidolon Ally to upgrade your rifle into the Necrochasm.