The Isu are the fictional precursor civilization that created the Pieces of Eden in Assassin's Creed.
They are an advanced species who lived on Earth before humans became dominant. In the series' mythology, the Isu created humans and used Pieces of Eden to control or influence them, including the Apple of Eden and the Staff of Eden.
A conflict involving the Isu, humans, and the misuse of these artifacts forms part of the franchise's modern-day mythology. Their civilization was devastated by a global catastrophe, after which only remnants, recordings, and artifacts remained.
The Isu were previously and sometimes informally described as the First Civilization or Those Who Came Before. “The Isu” is the later canonical term used throughout the franchise, so it is the most precise answer.