What is the name of the Templars' ancient rival organization in Assassin's Creed?

The Templars' ancient rival organization in Assassin's Creed is the Assassin Brotherhood. The Assassins oppose Templar efforts to impose order through control, surveillance, and powerful Pieces of Eden.

The Brotherhood's guiding maxim is “Nothing is true, everything is permitted.” In the games, this phrase is connected with freedom of thought and responsibility rather than unrestricted behavior. Assassins commonly work through secrecy, intelligence, alliances, and targeted attacks.

The organization has different names and local traditions across historical periods. Altaïr leads the Levantine Brotherhood in the first game, while later stories feature Italian, Ottoman, Colonial American, French, and other branches.

The modern Assassin Brotherhood also continues the struggle through descendants and allies such as Desmond Miles. The First Civilization is a separate precursor species, not the Assassins' organizational name.