Serapis Unites (Side Quest)
This is the first quest granted in the Sarapeion- a giant Temple to Serapis in Alexandria.
Activate the objective marker to pray, and fall into conversation with the priest, who needs your help.
His drummer has been wrongly imprisoned in the Kibotos Arsenal to the north- you need to retrieve him.
Since you're going to end up carrying him out, the best plan involves incapacitating everyone there first. And you might as well complete the location objectives while you're at it. As ever, make a careful scan of the area with Senu.
You'll see that the two treasures are toward the south side of the fortress, and only place really visible to passersby is the eastern gate. Be sure not to leave obvious bodies in this area, because passing riders and city patrols might see them and decide to investigate. Advance to nightfall and start your run in the south west corner. You can loot watchtower for arrows, but only if you're low- there are plenty other ammo caches in the Arsenal.
Peer inside. There's a little corner here where the soldiers go to piss- a stealth takedown here both removes a guard and gives you an easy way down.
Clamber up the ladder here for the first location treasure, in a red chest. No one patrols this particular path, but keep an eye to the east, where changing guards might be able to spot you.
Duck down the ladder to find both the second treasure and the commander's bed.
Check for other guards in this room. Take them down if present, then off the commander and loot the chest. Exit via the same ladder, and climb up to the top of the wall. Keeping to the wall, move clockwise around the fortress, taking out the largely inattentive guards.
When you reach the gate, you'll have a perfect vantage on the captain.
Wait 'til nearby riders have moved on, then take him down to complete the location.
This should leave just a handful of guards left at ground level, many of them asleep. Mop them up so you can have free run of the place.
Do use your pulse to reveal lootables- there are a quite few here.
When you're done with take, go to the center of the compound and unlock the drummer's cage.
Carry him out of the fortress, and keep carrying until you've officially left the conflict area (the game will let you know when).
Set him down and speak with him to get a lead some bandits in the market. Senu will show you where.
There are three. The goal doesn't make it explicit, but you pretty much have to kill these guys.
Engage as you like, then return to the Temple (fast travel is fine). Interrupt the conversation to complete this quest.