The Assassin Brotherhood's fictional precursor organization in Assassin's Creed Origins is the Hidden Ones.
Bayek and Aya establish the Hidden Ones in the aftermath of their fight against the Order of the Ancients. The organization is designed to oppose oppressive power while remaining concealed, and its creed and methods provide the foundation for the later Assassin Brotherhood.
Origins is a prequel to the earlier games, so the Hidden Ones are not yet called the Assassins. The distinction matters: the game portrays the group's formation rather than an already established medieval Brotherhood. Its members use hidden blades, secret cells, and coded practices that evolve across later centuries.
The Order of the Ancients is the opposing organization in Origins and is the predecessor of the Templar Order. The Levantine Brotherhood is a later term associated with the Assassin organization in the region of the Third Crusade.