The Hidden Ones are the ancient precursor to the Assassin Brotherhood in Assassin's Creed Origins. Bayek and Aya establish the group near the end of the game's story after opposing the Order of the Ancients.
Origins is set in Ptolemaic Egypt during the first century BCE, before the historical Assassin Brotherhood shown in later games. The Hidden Ones adopt principles and practices that anticipate the later Creed, including secrecy, resistance to tyranny, and the use of concealed blades.
The Order of the Ancients is the opposing organization in Origins and is the predecessor of the Templar Order, so it is a common mix-up. The Hidden Ones and the Order of the Ancients are separate groups with opposing goals.
Later historical storylines show the Hidden Ones developing across different regions and eventually becoming known as the Assassins. Origins therefore functions as an origin story for the series' central faction.