The mercenary outfit is a unique reward for completing the quest missing persons in Fallout 76 Steel Reign. That said, if you want the reward you will need to start by first completing the quest prior to missing persons and the entire Steel Dawn questline.
Once you did that, you will be able to start the missing person quest by speaking to scribe Valdez, and afterward, you will be tasked with locating a missing person. In the early portion of this quest, all you will need to do is talk to various persons at Fort Atlus.
Eventually, you will learn that Marcia, the person you are looking for, may have left to join a raider war party. When you learn this, the next stop will be too look for here over at The Crater, the raider-controlled settlement to the north of the map.
When you arrive at The Crater, you will need to visit the war room, and inside, you will find both Marcia and Pierce talking about two missing members of the raiders. After speaking to them, you will then need to go locate both of these missing raiders with Marcia over at the AMS HQ - you can find this HQ location in Watoga, so head there now.
That being said, when you arrive, you will need to head inside and go up an elevator, and when you reach the floor above, you will hear the sound of gunfire.
At this point, you will need to aid Marica with a band of Hellcat mercs and search for the missing raiders. To search for the raiders, you will need to head to the third floor of the building and locate a dove necklace and then a dead body on the top floor of the building.
Once you locate both the body and necklace, you will need to speak to Marcia again and then locate a handwritten note.
This note will then lead you to find a door on the top floor of the building that you will need to open with a nearby terminal. After this bit, you will soon need to fight a small boss battle that will lead to you to the basement of the building. In the basement, you will find both the missing raiders and can decide the fate of Marcia via dialogue options.
When you are done, return back to Fort Altus and turn the quest into Valdez, and you will get the mercenary outfit as a reward.
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