Another story related achievement so be aware of heavy spoilers!
*** Spoiler - click to reveal ***After reaching Alexandria you will be taken to see Aya, Bayek's wife. You have to kill two men who are hunting her, Gennadios and the Snake. Uncovering the Snakes identity is a mission on its own, but once both are dead and you return to her, there will be a loving cutscene and you will be sucked out of the animus.
It should pop around that point, but note that my achievement did not pop until I turned on the game the next morning so any comments on when exactly this should unlock would be greatly appreciated.
Let's keep cheevin' together!