The Hidden Ones are the secret society that preceded the Assassins in Assassin's Creed Origins.
Bayek and Aya founded the Hidden Ones after confronting the Order of the Ancients, a powerful organization seeking control over Egypt and its rulers. Their new group established principles and methods that later developed into the Assassin Brotherhood.
Origins is set in Ptolemaic Egypt during the first century BCE. Its story therefore shows the formative stage of the organization rather than the medieval Brotherhood seen in the earliest game.
The Hidden Ones should not be confused with the Order of the Ancients, which serves as their principal enemy in Origins. The modern name “Assassins” and the recognizable Creed emerge later in the franchise's fictional chronology.