No Chains Too Thick
This story mission pops automatically once you've killed all three of the evil lieutenants marked by the Hidden Ones. Return to Tahira in the Bureau for the final phase.
During the cutscene, the Bureau is attacked, and Bayek winds up in a bad way.
After being rescued, it's straight back to work, saving the two other fire survivors from Fort Clostra.
Consider just killing everyone here- you're going to have to carry Tahira out the far side of camp, and the prisoners here are civilians, not rebels, who will tend to throw themselves on the nearest enemy weapon they can find. Start by crossing the fence nearest to the crucifixion site.
Assassinate the near guards, then make your way up and along the watchtowers.
Snipe/sneak-kill target of opportunity, but try not to let loose until you've trapped the braziers (extra guards aren't really a threat to you, they're just tedious).
You can also use the central mounted crossbow- it does extremely impressive headshot damage.
While you're killing, make a point of snuffing the local officers as well. The camp commander tends to hang out by (and sometimes behind) his tent on the south easterly edge of camp.
That same tent contains one local treasure in carbon crystal form
The other treasure is in a large tent just southeast of the crossbow tower, in a red chest.
The local captain likes to patrol the top of the hill, next to the tent where Tahira is being held.
When ready to begin the liberation phase of the mission, find Kashta in the lion pit.
He's well enough to simply follow you to the meeting point west of camp.
When you've cleared the camp of hostiles, free the civilians, then carry Tahira to safety.
Mount up outside of camp. There will be one small ambush on the way, by a Roman banner and some ruined pillars. Snipe or fight your way through them to continue (there is a arrow-refill here if you need it).
Reach safety to trigger a cutscene. The mission ends with it.
This automatically triggers the mission Sic Semper Tyrannis.