In Assassin's Creed Mirage, Basim joins the Hidden Ones before becoming a full member of the organization.
The Hidden Ones are the historical precursor to the later Assassin Brotherhood. Mirage follows Basim Ibn Ishaq in ninth-century Baghdad, where he begins as a street thief and eventually travels to the Alamut stronghold to train under the Hidden Ones.
The game's story explores Basim's initiation, his training, and his missions against the Order of the Ancients. It presents a younger Basim than the character seen in Assassin's Creed Valhalla, where he has a major role during the Viking Age.
The Order of the Ancients is the opposing organization in this period, while the Templars emerge as a later historical form of the series' rival faction. The name Hidden Ones is therefore specific to the early era depicted in Mirage.