Aya Part I
This part of the main quest requires no fighting, but will see Bayek gain his assassination blade, and lose his beard. Once you've steeled yourself for these changes, go the Library and follow your marker to the statue of Serapis.
A confederate of Aya's will lead you out and to a well with a secret passage. There's no ground-floor entry, so climb up and over, then dive into the well.
Cutscenes will catch you up on events here, and grant you a piece of history in the first Assasin's blade- the one that slew Xerxes. When all is done, you'll have two new quests, and opportunity to loot this tiny room.
Exit via the surprisingly modern-looking prison door when ready.