To reach Chenzira and find the ancient tablet inside Siwa’s Mountain of the Dead in Hideaway in Assassin’s Creed: Origins hit the wall at the back left of the tomb:
Go through the room ahead and turn right at the top of the slope. Go up the slope and turn left. In the room ahead destroy the pots behind the rack on the left wall:
Then push the rack backwards and go through the opening. Drop down and Chenzira is standing in front of the ancient tablet.