Perhaps the most infamous weapon in all of Destiny, the Year 3 Gjallarhorn rocket launcher is also the most sought-after Exotic Weapon in Rise of Iron. Here's how you get the Year 3 Gjallarhorn.
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Start a Patrol in The Plaguelands and head to Archon's Keep. Look for a mission beacon with a ghost icon and start the “Locate and Analyze the Anomaly” mission, which will take you through Archon's Forge and The Warrens all the way to Site 6. If you need help finding the Patrol mission beacon, check out Plaguelands and Quarantine Patrol Hints for what symbol you need to look for. During this Patrol, you'll need to get through a Splicer Gate that requires a Splicer Key. If you don't have one, read How to Get and Use Splicer Keys, Splicer Intel Relays, and SIVA Offerings for instructions on how to get one. Continue on until you make it to a Splicer Walker, which you should destroy before you start scanning. Aim for the red parts of its legs to do massive damage to it when a leg is destroyed.
After scanning the anomaly, you'll be called back to the Iron Temple. Talk to Lord Saladin and Tyra Karn to get the “Beauty of Destruction” quest, where you have to find seven Iron Medallions scattered throughout the expansion areas. For the locations of each of the seven Iron Medallions, check out our guide “How to Find the Iron Medallions”.
Once you've found all seven Iron Medallions, report back to Tyra Karn, who will give you a mission at Bannerfall. The mission itself is fairly straight-forward, but the part where you have to disarm two bombs might give you difficulty, especially if you're playing solo. Disarming the bombs means staying inside the glowing circle until your Ghost is done disarming. The difficulty comes from the waves and waves of Shanks that swarm you at every turn, particularly the Splicer Exploder Shanks. You absolutely want to keep your distance from their kamikaze attacks, and when you destroy them, try to catch as many enemies in the resulting blast as possible, even chaining multiple Exploders at once. The very end of the mission also throws a boss enemy at you, Iliksis, Light Stalker. Iliksis is tough, but you can bait him into focusing his firepower at the automated turrets you turned on just before the bombs were armed. They'll do some damage, but they mostly draw his attention, letting you concentrate your firepower on him.
Return to the Iron Temple and talk to Tyra Karn. She'll then task you with finding five Dormant SIVA Clusters. For their locations, check out our guide “How to Find All Dormant SIVA Clusters (Coming soon!)”. Return to her after, then speak to Lord Saladin to get the “Beauty in Delivery” mission, where you'll have to eliminate a horde of Fallen, then defend yet another point while your Ghost works. The effective area you can be in for the Ghost's percentage to keep going up is much larger here than in the previous mission, so you actually have the space to run around the point having enemies pursue you, only needing to stop and kill the stragglers who break from the ranks and end up in your way.
Once you've fully defended the point, you'll finally receive your very own Year 3 Gjallarhorn, which you'll then need to use to kill two Splicer Walkers to complete the mission.