Completing all six codex pages
After obtaining all six codex pages, return to your settlement so you can speak to Hytham.
He tells you that he can’t decipher the codex pages at all. Instead, Reda, the leader of the Thousand Eyes, has to do it.
You might need to upgrade your settlement level first for him to appear.
Anyway, Reda tells you that the pages were given to him by a friend, Bayek, someone he hasn’t seen in a long time. Oh, the plot thickens.
Check out the deciphered letter inside Reda’s tent. You’ll hear Bayek narrating the contents of the letter. That completes “A Brief History of the Hidden Ones.”
Note: Although Reda bugged out for me, the quest allowed me to speak with him briefly. Unfortunately, I couldn’t speak to him again after reading the letter and finishing the quest. Hopefully, you don’t encounter this issue in Assassin’s Creed Valhalla.