Shield or Blade (Side Quest)
Launch this sidequest by speaking to a blind oracle in the Duat realm.
She tells you to seek out a forbidden tablet, which is relatively easy, because it's in the building right behind her.
Once inside, go to the door at the south end of the hall, and find it locked.
As usual, look up for a high window to parkour through.
Use the gold marker to identify the Heretical Texts (which can't decide whether they're a scroll or a tablet). Note also the stelae here, if you haven't already read it.
Bring the texts to the oracle to gain three miniquests, recalling the purification rituals of the Aaru realm. These quests are Khepri's Amulet, A Necessary Evil, and The God's Spark.
When you've completed all three, meet up with the oracle near the entrance to the Duat Realm. Continue to place an astonishing amount of trust in a dead sorceress you've never met by interacting with the flame to begin the ritual.
Anubis will send a few waves of dog soldiers after you- less than a dozen total, but all of them near the level cap. If you've already maxed out this is an easy fight, but lower level Bayeks may want to use the altar as obstacle/cover.
Wiping out the jackalmen leads to an odd cutscene. First you destroy the soul of Septimius (who's been dead for at least four years- how is he just crossing over now? Given his unexpected (to him) demise, where did he get a child's heart? Did Anubis not think it odd that a giant man could have lived with such a diminutive cardiac musculature?). Then the oracle assures Bayek that having personally killed thousands of people, wild animals, and assorted spiritual entities won't stop his own soul from reaching paradise. Which is nice? Regardless, it completes the mission.
In addition to a free pass into the afterlife, Bayek also gets the Servant of Amun outfit.