The First Civilization in Assassin’s Creed mythology is called the Isu.
The Isu are an ancient, technologically advanced species that existed before recorded human history. In the series, they created humanity and many of the artifacts known as Pieces of Eden, including devices capable of controlling or influencing human minds.
The Isu are also called the Precursors in some Assassin’s Creed material. Their civilization declined after a catastrophic conflict involving humanity and a solar flare. Survivors and remnants of their technology continue to shape the modern and historical storylines across the games.
The Isu are not the same as the Assassins, Templars, or Hidden Ones. Those are human organizations that emerge much later. Names such as Minerva, Juno, and Aletheia refer to individual Isu figures or entities, while “the Isu” identifies the civilization as a whole.