The Hidden Ones are founded in 47 BCE in Assassin's Creed Origins.
Bayek and Aya establish the Hidden Ones after the events of the game's main story. Their goal is to protect ordinary people from oppressive power while remaining concealed, creating the organization that later develops into the Assassin Brotherhood.
The date is connected to the closing events of Origins, which take place during the political struggles of Ptolemaic Egypt and the Roman expansion under Julius Caesar. The founders use a creed centered on staying hidden and serving the innocent, ideas that echo through later Assassin traditions.
The Hidden Ones are not the same name as the later medieval Assassins, although the game presents them as their predecessors. The order they oppose is the Order of the Ancients, which predates the Templar Order. Dates such as 44 BCE are tempting because they mark Julius Caesar's assassination, but that is not the founding year given for the Hidden Ones.