The Isu are the fictional precursor civilization whose artifacts appear throughout Assassin's Creed.
The Isu, also called the First Civilization in earlier terminology, existed on Earth before modern humans. They created powerful Pieces of Eden, including the Apples of Eden, and engineered humans for labor. Their conflict with humanity and their attempts to survive a global catastrophe form a major part of the series' deeper mythology.
Artifacts associated with the Isu influence historical events across many games. Characters such as Altaïr, Ezio, and Desmond encounter or inherit the consequences of these objects, even when they do not understand their original creators.
The Templars and the Order of the Ancients are human organizations, not the precursor civilization. “First Civilization” is a descriptive label used in the series, while Isu is the civilization's later and now standard name.