Assassin's Creed calls its advanced precursor civilization the Isu.
The Isu are an ancient species that existed long before recorded human history in the series' fictional universe. They possessed highly advanced technology and created powerful artifacts known as Pieces of Eden. Human societies later remembered some Isu figures as gods or legendary beings, including Juno, Minerva, and Jupiter.
The games gradually reveal Isu history through holographic messages, temples, genetic memories, and modern research. A major part of the mythology concerns the Isu creation of humans and the conflict that eventually led to a human uprising. The Templars and Assassins interpret Isu artifacts differently, but both factions seek their extraordinary power.
Older games often used terms such as “First Civilization” or “Those Who Came Before.” These phrases describe the same precursor culture in context, while Isu is the later and now standard name used throughout the franchise. The Isu are fictional and are not identical to any real-world ancient civilization.