The legendary First Civilization in Assassin's Creed is called the Isu.
The Isu are an advanced civilization that existed before recorded human history in the series' fictional mythology. They created the Pieces of Eden, including the Apple of Eden, and engineered humans for labor and service. Their society was eventually devastated by a catastrophic solar flare.
Although most Isu died before the main games, their technology and surviving memories shape the conflict between Assassins and Templars. Characters such as Minerva, Juno, and Jupiter appear through messages, simulations, or artifacts connected to this earlier civilization.
The Isu are also called the Precursors in some Assassin's Creed material, but “the Isu” is the principal name used by the games. They are not simply another human empire or a conventional group of gods; the series presents them as an advanced species whose technology inspired later myths.