The Walls of the Ruler
This is one of the story missions acquired right after speaking with Tahira, during your first visit to the Bureau.
When ready, make for the easily-spotted watchtower southeast of Klysma Quarry.
This will take you to the rebel encampment- Paratiritirio.
Speak to your contact atop the tower for the lowdown on Ampelius.
Ampelius is dressed like his captains, so Senu can't locate him. However, once you get close, Bayek will automatically recognize him, and mention it. A 'Target Located' will also flash onscreen.
Even before that, you'll hear officers updating Ampelius with new information, shouting things like "We've captured a rebel, sir!", "We've executed the suspects, sir!", 'The Hidden One has been dealt with , sir!", etc. Amelius always travels with a retinue of guards, and can appear anywhere captains appear. The guards will stay with his body rather than getting drawn into nearby fights, so it's best to slay them all at once. One good ambush point is in the southeast corner of the fort, near one of the captain offices. You can sneak inside, smash through the roof, or ambush him as he emerges.
Do not follow him into the dog arena, through. The captain there is tougher than pretty much all the other warriors in the fort put together, and is best dealt with separately.
There is some randomness here due to the constant rebel attacks so don't get too hung up on a swiss-watch style assassination. If you're going for total completion, it's probably most efficient to make a go at clearing the fortress- there are six treasures to loot here, and you'll be carrying a body out of the place for a sidequest. Start by working your way around the upper parapets, clearing guards and trapping braziers.
When you can move around the walls comfortably, snipe or dropkill targets of opportunity. You can also score easy XP by sniping anyone fighting a rebel.
When the place is looking reasonably deserted, kill Ampelius and his guards (once you've ID'd him, the quest marker will continue to move with him), and confirm the kill.
After an extremely brief and surreal cutscene, the mission completes.