The Hidden Ones are the organization that becomes the Assassins in Assassin's Creed Origins.
Origins, released in 2017, depicts the formation of the Hidden Ones in ancient Egypt. Bayek of Siwa and Aya establish the group after confronting the Order of the Ancients, a secret organization seeking political power and control. The Hidden Ones develop principles and methods that later define the Assassin Brotherhood.
The game therefore serves as an origin story within the franchise's fictional history. It explains why the organization operates from the shadows and why its members oppose domination by powerful rulers. The Hidden Ones' activities also establish the symbolic tradition associated with the later Brotherhood.
The Order of the Ancients is the opposing faction, not the precursor organization. The modern name Assassins is used for later groups, including the Levantine Brotherhood seen in the first game, so the historical progression matters.