The Assassins are the secret order opposing the Templars throughout the Assassin's Creed series.
The series frames the conflict as a struggle between the Assassins, who defend free will, and the Templars, who generally seek peace through control and imposed order. Individual characters and factions complicate those ideals, but this ideological rivalry is the franchise's central historical conflict.
The order uses the name Assassin Brotherhood in many stories and traces its fictional lineage through groups such as the Hidden Ones in ancient Egypt. Altaïr, Ezio, Connor, and many other protagonists become members or allies of the Brotherhood in different periods.
The Hidden Ones and the Order of the Ancients are earlier historical organizations that later evolve into the familiar Assassin and Templar traditions. They are related to the same long conflict but are not the standard name of the recurring opposing order.