The secret First Civilization in the Assassin's Creed universe is called the Isu.
The Isu are an advanced species that existed on Earth before modern humans. In older games they are often called the Precursors or the Ones Who Came Before. Their artifacts, including Pieces of Eden, influence the central conflict between the Assassins and Templars and explain many of the series' seemingly supernatural technologies.
The Isu created humans as a labor force, but human rebellion and a catastrophic solar event changed the course of both species. Only limited records and surviving artifacts remain, so the games reveal Isu history gradually through visions, recordings, genetic memories, and archaeological discoveries.
The Templars are a modern human order, while the Hidden Ones are the early Egyptian organization that evolves into the Assassins. Neither group is the First Civilization. Later stories introduce individual Isu figures, including Juno, Aita, Minerva, and Jupiter, but “the Isu” is the collective name for the civilization.