The Gjallarhorn Exotic rocket launcher is back for Destiny year three and on the long road to getting your hands on it, you'll be tasked with snuffling out some collectibles.
We're here to help you out with that part, but if you're going solo, the rest of the questline won't be so easy. Good luck - you're going to need it.
With the exception of the Iron Temple, you'll find them all in the Plaguelands Patrol.