This gun is not only available in the demo, it is not only available to low level either. I obtained the Husk of the Pit in the full game, while playing as my level 31. Read the description of my video and you'll see how I got it. If you want proof that I have it, SEND A MESSAGE TO INSANEBIKER118 AND I WILL INVITE YOU AND SHOW YOU.
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It's stuck as a common until you reach eris lvl 3 and buy a core to make it legendary and then to make it the necrochasm you need to complete the dark below raid on hard