The Isu were the precursor civilization in the Assassin's Creed universe.
The Isu, also called the First Civilization, were an advanced species that existed before recorded human history. They created powerful Pieces of Eden, including the Apple of Eden, and engineered humans as a labor force. Their civilization was devastated by a solar flare and the resulting catastrophe, after which humans became dominant.
“First Civilization” is a descriptive alternative name for the Isu, not a separate group. The Templars and the Hidden Ones are human organizations that appear much later. The Isu's surviving technology and genetic legacy drive much of the modern-day and historical conflict across the franchise.