Nova Mortis Exotic machine gun
The Nova Mortis is the Void variant of the Thunderlord heavy exotic and you'll need to initiate a quest chain to get it.
First off, you'll have to get to level 7 in the new Competitive Spirit book to get Xur's Tag.
Head up to the platform that holds the Dawning gifts at the far back of the Tower and use the tag to open Xur's Gift in the centre. This will kick off the Songs from the Void quest.
Take the schematic from the box to Ikora Rey, who will then direct you to Banshee-44.
Banshee-44 will send you off to get 100 Void ability kills. Return to him again once you're done.
He's not going to hand over the gun just yet. Boot up The Summoning Pits strike and crank it up to hard difficulty to play The Abomination Heist that Banshee-44 wants you to do.
Once you've finished the strike, visit Ikora Rey to get your gun.