Egypt's Medjay (Main Quest)
Use Senu to ID your contact outside the Hippodrome.
Speak with him to get a rendezvous with Apollodorus- he'll meet you at the Kanopos Nome lighthouse, but only at night. Luckily the light house is also a fast travel point.
Show up after dark to meet the man himself.
You'll chat, and be given an objective to rescue an agent held on a trireme across the bay.
The sneakiest way through is from the landside – there's a rope running across the street from the stables that will take you all the way to the crow's nest of the target ship.
You're going to have to fight your way out, so you may want to do some prepatory killing while you're up here.
When ready, free the captive to begin your run.
More soldiers will appear on shore- fight them off. As usual , you may get warning about an ally's low health, but the captive can't be killed or recaptured, so don't sweat it.
When you gain enough distance, talk to your rescuee. He'll reveal that the scroll is on a boat, and the boat has been sunk.
The wreck isn't far, grab your own boat from the dock, and use Senu to pinpoint the objective spot.
You won't be able to dive straight down- you'll have to swim inside the severed boat shell to retrieve the scroll.
Meet Apollodorus in the indicated marshlands, and give him the papyrus.
Return to Apollodorus' villa. Take any path you like- he's up for it, and there are no enemy encounters en route.
Arriving begins a long cutscene which reveals more about the Order and the Snake, and gives Bayek a whole new slate of masked men to slaughter.
You regain control the next morning, with mission completion.