Assassin’s Creed uses the name Isu for its precursor species.
The Isu are an advanced civilization that existed on Earth before modern humans. Earlier games often called them the First Civilization or Those Who Came Before, but later lore commonly uses “Isu” as the species’ proper name. They created powerful Pieces of Eden, including the Apple of Eden, and engineered humans for labor and service.
A conflict between the Isu and humans culminated in a catastrophic solar event. Although much of the Isu civilization was destroyed, surviving technology and records shaped later human history and the franchise’s central mythology.
The Isu are not the same as the Templars or the Assassins. Those are human organizations that arise much later. The Hidden Ones are Bayek’s ancient organization in Origins, while “First Civilization” is an older descriptive label for the Isu rather than a separate species.