Talk to Ravan in Meridian to start this quest. He’ll tell you that he’s been robbed and that his family heirloom has been stolen. First things first is to talk to his Steward, who won’t be able to give you much new information, so instead use your Focus and scan the nearby window to pick up the thief's tracks.
Follow the tracks and they’ll lead to a merchant who’ll tell you where the thief went after he reached her stall. Continue following the tracks until you hit a dead end, then use your Focus to find Rokasha, a woman nearby. She’ll tell you that the thief didn’t have the sword when he passed by.
So it’s back to the merchant, who was actually in cahoots with him. She had good reason however, and she send you down to the village below Merdiain to find out what it is. Head down the lifts to the mill, where you’ll be told the reasons behind the robbery. Now it’s a case of finding the original thief. Go the waypoint marker at the Branded Shore and then use your Focus to investigate the scraps of cloth to find the tracks. Follow them to the Carja camp where Nasan is being held.
These guys are heavily armed, and armoured, so try not to get into a stand up fight with them if you can avoid it. The gateway acts as a natural choke point, so you can easily set a bunch of tripwires and traps to thin out their numbers, then you can just play hide and seek with the rest to take them all out.
When they’re all down, speak to Hasan. Make sure you loot the supply crates that are nearby as well as they will have some nice prizes. Follow Hasan up the walls and then rope off the ledge. When you land, he’ll head off and then all you need to do is head back to Meridian and talk to Ravan to finish the sidequest.