Blood in the Water (Main Quest)
This picks up the thread of the Main Quest after you've searched Tychon's Villa. Head south to the town of Swenett (expect some bandit attacks en route), and seek out the scribe's house, only to find no one home.
You'll get directions from an eccentric passerby. Scanning the target area with Senu shows an ambushed cart.
Investigate the fallen body, then follow the trail to the nearby bandit camp- the Flooded Farm Hideout.
The loot you need is not in a particular place, but is carried by a particular person. Watch for the person with a gold plot marker floating over their head, then kill and search them (bribery, intimidation, and persuasion are not offered options).
The manifest points you to an anchored ship. Expect even more bandit ambushes en route, though some you can see coming.
Speed on out to the boat when you reach it, and don't worry about hawksight showing hostiles on board- it's just vultures feasting on all the corpses.
Investigate the boat, but the key clue is the spot where someone was dragged overboard. Note the blood trail leading to shore.
Snipe the local waterbeasts before following that same trail yourself.
On shore, speak to the man in the loincloth to learn of survivors.
Follow his clue to the Moon Curser village.
This will lead to a reunion with Sutekh, who describes what happened on the boat, and points you to the real operator, Merti.
Before going after Merti, note that her sequence is going to end with a level 55 Shadow of Anubis confrontation. It's a fight you can flee, but if you feel like you just need another few XP or some upgraded gear to handle the fight, now is a good time to make those preparations. When ready, follow your marker to the House of the Blue Lotus.
No sneaking around this time- all these guards have to die. Note that, if you've been upgrading your quiver, you have about twice as many predator arrows as there are hostiles.
When they're all dead, approach the wooden door with the quest marker to begin a long conversation with Merti.
When this conversation ends, the Shadow appears. He starts out at quite a distance, so it's possible to flee before you're even spotted.
If you do choose to fight (and this is a nice, isolated area with few distractions), it's actually not so bad, as mini-bossfights go. His moves are pretty much the same as one of the oval-shielded commanders, and he doesn't have any magical healing powers or area attacks. Akhenaton's sword is very effective against him, and looping overpower-slash-slash again and again will quickly wear him down to zero.
If you do kill him, remember to search the body before leaving.
Now the trail leads to the Necropolis of the Nobles.
Approach the location for an automatic cutscene, which reveals Matri's source to be Tahemet.
This next bit can be a little confusing- Tahemet will walk you to the tunnel entrance where you can continue the Blood in the Water quest, but this also spawns the completely unrelated King of Kings side quest. If you track that (perhaps thinking it's a continuation of the main quest), your immediate objective is to talk to Tahemet, only you can't, because she'll be 'unavailable' until she returns to her shack, and you've left her alone for a bit.
The key point is don't activate King of Kings yet- even if you're bored with the main quest, you can't start King of Kings right away anyway.
Moving on, the tunnels are a bit of maze, but your goal is to find the four murals which comprise the ritual. They'll be highlighted with gold markers in your Senusight. One path is break open the first scarab wall you come across (it'll be on your left).
Go down the stairs, and you'll be in the room with the 'missing rays' mural (there is also a treasure in the room just east of here)
Bust through the smashable wall to the northeast, and use it as an ambush point for nosy hostiles.
When the immediate area is clear, use the coffins for cover as you bear northwest.
This long room contains the Amun mural (and a location treasure in a white chest).
Go east out of this room, and bear left until the passage ends with ladder leading up.
Ambush the guards up top. Just by the ladder is the descendants mural.
On this same level, follow the passage south to the final orb mural.
This allows Bayek to put all the pieces together, completing the mission and launching the Motherless Child quest.