Bayek helps establish the Hidden Ones in Assassin's Creed Origins.
Set in Ptolemaic Egypt, Origins follows Bayek of Siwa and Aya as they respond to political violence and investigate the Order of Ancients. By the end of the story, Bayek and Aya create the Hidden Ones, an underground organization devoted to opposing oppressive power while remaining unseen.
The Hidden Ones are presented as the direct predecessor of the later Assassin Brotherhood. Their creed and methods explain how the series' familiar organization develops from a smaller movement into a network that appears across subsequent historical periods. The game also links their emblem and practices to the origins of the iconic Assassin symbolism.
The Assassins is the later, more familiar name of the organization, so it is not the precise answer for Origins' founding moment. The Order of Ancients is the opposing faction, while the Initiates are a separate modern-era term used elsewhere in the franchise.