Did you even read the patch notes? They say why they nerfed it. " Gjallarhorn If Destiny had a nuke it would be the “Ballerhorn.” We definitely intended to have a high damage Heavy Weapon that was ideal for PvE destruction. What we did not intend, and what we unfortunately saw, was pick up Raid and Nightfall groups gating participation based on whether or not players had this weapon. Gjallarhorn was so strong that for many people it had become the only answer to getting through tough encounters, and therefore they were less willing to spend time with other players that didn’t have it. We strive for Destiny to be a place where a single weapon or strategy does not dictate how, or with whom, you spend your time. In the new world Gjallarhorn is still worthy of its legacy as an exotic Heavy Weapon, but we hope it promotes inclusive behavior rather than exclusivity."