Pissa Oros Citadel (Location)
A large enemy complex in the Isolated Desert. You'll pass through during the Plight of the Rebels side quest.
It's a giant place, so scan thoroughly, but luckily all the treasures are grouped together in the central building.
A good access point is the passage under the southwest arches.
Climb in and up to emerge near the two major south parapets. One of the captains patrols around here, and it's an ideal time to take him out (along with nearby targets of opportunity).
The second captain also like to hang near here, on the other side of the central causeway, but he can't be taken out so easily.
As one of those phylakes-tough officers, your best best is to lure him into the darkest part of the training yard, where you can take him apart with causing to much ruckus. And frankly, he's probably glad to be free of his unspecified but clearly monstrous and untreatable glandular disorder.
Stash the body and clear nearby targets, then head for the central building.
Dispatch the courtyard guard, and stalk/snipe any wall guards who take notice.
Enter the commander's quarters for an easy backstab.
Loot the white chest in these rooms, and the carbon crystals on the side table.
Exit back into the courtyard, and sneak up the big stairs to your right. Follow them up to the watchtower, where a red chest waits.
Cross the walkway northwest to the next watchtower, and loot the small red chest there to complete the location.