Assassin's Creed calls the ancient advanced civilization that existed before recorded human history the Isu.
The Isu are an ancient species whose technology and genetic legacy shape the series' mythology. Earlier games often called them the First Civilization or the Precursors, but Isu is the name used in later lore and reference material. They created powerful artifacts, including the Pieces of Eden.
The Isu ruled Earth before a catastrophic conflict with humanity and a solar event devastated their civilization. Survivors and remnants influenced later human cultures, while their artifacts continued to drive the Assassin-Templar struggle. Characters such as Minerva, Juno, and Jupiter are associated with the Isu storyline.
The Isu are not the same as the Templars or the Hidden Ones. Templars are a human organization, and the Hidden Ones are the ancient Egyptian group that develops into the Assassin Brotherhood. The term First Civilization remains a common answer in older discussions, but Isu is the specific canonical name requested here.