Rise of Shaqilat
Speak to the bearded acolyte in the Hidden Ones' Bureau to begin this quest.
His prompt will lead you to some tangled merchant ships just offshore. Despite his description, however, there are no soldiers here. At least, no live ones.
Board the ships (you can jump from a tree to a sail, or just swim over) to enter an investigation area. Most of the markers are cages, but the lead ship has a robbed chest and slave ledgers.
Completing the investigation leads to the Underground Outpost location in the quarry.
Use the rope stretched above the quarry entrance to dropkill the outer guard (the inner guards can't see this far).
There are three guards inside. Since you'll have to walk the captive out in a moment, all three need to die. You can snipe or slash as you prefer, though note the oil-jar/hay-mat near the entrance make a dandy ambush spot.
When they're dead, loot the carbon crystals from a mat next to small bench in the back room to complete the location.
After, you might as well free the prisoner you came to rescue.
There's no new opposition waiting outside, just walk with him to the ledge and speak when prompted.
These words lead you to Shaqilat herself, playing Robin Hood at least a millennium before it was cool.
Speak with her about Roman reprisals, and lo, Roman reprisals appear, in the form of two large gladiator-level warriors, miscellaneous archers & soldiers, and a ninja.
Shaqilat will fight with you, and though she's unlikely to rack up any kills herself, she can keep some of the enemies occupied while you work. A good strategy is to use your starting adrenaline on the shield-carrying gladiator, then use arrows to take out the mere mortals (starting with the ninja) before coming to Shaqilat's aid.
When the fight's over, follow Shaq over the wall, and make your recruitment pitch.
When you're done talking, perform a Leap of Faith off the white-marked pole (as per usual), then watch the cutscene to win over Shaqilat, and complete the mission.