Now that Destiny's Age of Triumph is live, there's a whole bunch of 'lost' guns that have been brought up to year 3 level (400 light). The Necrochasm is one of them and features Cursebringer - a perk which causes a cursed Thrall explosion with every kill. Beautiful.
To get it you'll need to activate the Husk of the Pit quest. I got it during a mission from the story playlist but it's also been reported to drop after Nightfalls or from random Ogre kills. Once the quest drops, you'll be given the Husk of the Pit auto rifle. The first thing you'll need to do is upgrade this to the Eidolon Ally by killing Thrall and in particular, Wizards. So pick your favourite Thrall infested mission (which is obviously The Dark Below) then repeat. Once you've done this a few times, you should have enough kills to light up the Cannibalism slot in the Husk of the Pit's upgrade tree.
With this done, pay a visit to the Speaker in The Tower where he'll upgrade the Husk and turn it into the Eidolon Ally. Next, take this to Eris, who's helpfully also in The Tower (seriously, why don't these guys just talk to each other once it a while) and she'll activate the next step in the quest. This will require a fireteam as Eris will ask you to complete the Crota's End Raid and then finish the Will of Crota Strike - that's the one with Omnigul.
Once you've completed the Raid and the Strike, pump all your motes of light into the Eidolon Ally to fully upgrade it and take it back to Eris. She'll reward all your hard work with the Essence of the Oversoul, which you can use to transform the Eidolon Ally into the Necrochasm - and you're done.
Looking for help with the rest of the expansion? We have a Destiny: Age of Triumph quest guide here. And if you're ready for the next chapter you can place your Destiny 2 pre-order here.