From Destinypedia, the Destiny wiki
The Nova Mortis is an exotic machine gun introduced in Destiny: Rise of Iron. Designed by an unknown Titan during the Great Disaster, it was designed to be a Void-based variant of the Thunderlord, only to be lost in the aftermath of the campaign.
The Nova Mortis is, simply put, a Void version of Thunderlord. It has the same designs, statistics, and perks as Thunderlord, and functions exactly alike.
List of appearances