Playing With Fire (Side Quest)
You'll unlock this one during The Final Weighing, as you chase Flavius across the land. Begin the quest proper with a jailbreak from Kyrenaika Citadel.
You don't have to infiltrate it just yet, however. The prisoner you need to free is in a prison yard outside the walls, southwest of the fortress.
Sneak in by hugging the fortress wall.
Note that this will become a fight-your-way-out type situation, so you can choose whether to stealth-kill your enemies now, or attack them with the help of all the prisoners you're about to free. When ready to begin the team section of the jailbreak, drop down and free Kade.
He'll prompt you to rescue the other prisoners, who will become highlighted by quest markers.
Free them, cut down any nearby guards (even open combat won't draw any attention from the main fortress), then make good your escape. There's a break in the fence due south that's perfect.
When you're clear of the fort, speak to Kade for a link back to Vitruvius.
Find your Roman chum back in his tent. Speak to him, and he'll lead you back toward Kyrenaika fortress, as he explains the situation.
There's no pickpocketing in AC:O, so you'll have to kill Agrippa for the plans. He hangs out in the upper yard of citadel, day and night.
There is a secret entrance which will get you close. From where Vitriuvius left you, just head toward the citadel, and dive into the watery pit (the one with the secret entrance icon).
Follow the tunnel to the burial pit.
You'll come up in the middle of the Citadel, close to the yard. But that's just half the problem- Agrippa is one of those phylakes types who simply cannot be one-shotted or stealth-killed- It's going to take time to eliminate him, and it's going to make a lot of noise. Also, you'll need to confirm the kill after, which is hard if an entire citadel is stabbing you.
Your best bet is to lure him inside one of the low buildings here- with a whistle or an arrow, then take him out in relative seclusion.
Adrenaline accelerates the process admirably.
Confirm the kill to recover V's formula, and get a Legendary shield.
Now you have to destroy three caches of Greek fire around the fortress. Note that it is safe to smash them with melee weapons, which is handy, because fire arrows won't even scratch them.
One of the caches is on the east side of Agrippa's favorite yard, right next to the middle gate (there's a location treasure here as well, if you haven't already gotten it).
The next is in the northeast part of the fortress, in a building next to a training yard.
The last is in a building near the center of the fort, east of the middle gate.
When the last jar is smashed, fast-travel back to the aqueduct, and update Vitruvius to complete the mission
Note that this is one of the three missions required to unlock the Pax Romana side quest.