The secret precursor civilization in Assassin's Creed is called the Isu.
The Isu are an advanced civilization that existed on Earth long before recorded human history. Earlier games often refer to them as the First Civilization or Those Who Came Before. They created powerful Pieces of Eden, including the Apple of Eden, and engineered humans for labor and service.
The series gradually reveals that many myths and religious legends are connected to Isu individuals, technology, or events. Their conflict with humanity and the catastrophe that nearly destroyed their civilization shape the modern Assassin-Templar struggle.
The term Isu became especially prominent in later games and official series material. It should not be confused with the Assassins, Templars, or modern researchers who study Isu artifacts through the Animus.