Talk to Yan in Mother’s Rise to start this sidequest. Head for Mother’s Crown and talk to her Aunt, who’ll tell you to head for Daytower, the same place you need to go for the main quest. You’ll need to complete, or have completed, the Seeker at the Gates main quest before you can continue this quest. Once you get to Daytower, speak to the commander of the fort, who’ll tell you that Zaid, the man the daughter is trying to kill, is there. Speak to him, then head to find the woman herself at Lonesome Rock.
Make your way to Lonesome Rock, where you’ll meet some Carja guards, who after a quick chat will quickly reveal themselves to be pretty awful people. Kill them all then use your Focus on the cart in the courtyard and investigate it to find the tracks from their transporting prisoners. Follow the tracks until you reach the broken down fort. Inside are a number of patrolling guards, but there’s plenty of opportunity to stealth around them and take them out at your leisure.
When they’re all dead, head up the main house and on the right hand side there will be an open door with a staircase leading down to a basement. Go down there to find Nakoa as well as two other prisoners. Talk to Nakoa to free her and the other prisoners, then head upstairs to find that Zaid has followed you and is none too happy about you busting up his slave trading. This will trigger a fight with him and a number of guards. The guards are simple enough, you can either stealth around them or just overpower them. Zaid is a little trickier, his Firestriker weapon does a lot of damage, and we at least couldn’t seem to knock him down or get a prompt for a stealth kill. So it’s a case of staying light on your feet and shooting him in between volleys from his weapon.
When he’s down you’ll get another cutscene then the quest will be complete.