History of Acosmic

Overview

Acosmic is a Legendary Void Grenade Launcher with a Rapid-Fire Frame (150 RPM, 5-round magazine, per Destinypedia and Bungie's item page), made by Nadir. Its listed trait perks include Field Prep, Wellspring and Destabilizing Rounds, a mix of ammo economy, ability energy and Volatile rounds. Its lore follows Deputy Commander Sloane at a Festival of the Lost celebration.

Acquisition

A Festival of the Lost weapon (Bungie source: Festival of the Lost). Bungie's item page also lists a shaping/enhancement socket, but I could not confirm that it is craftable; no Adept version was seen.