Assassin’s Creed calls its fictional precursor civilization the Isu.
The Isu are an advanced species that existed on Earth before modern humans. Earlier games often referred to them as the First Civilization or Those Who Came Before, while later franchise material commonly uses the name Isu. They created humans and many powerful artifacts known as Pieces of Eden.
The Isu storyline explains why objects such as the Apple of Eden can influence human perception and behavior. It also provides the science-fiction foundation beneath the series’ historical adventures, linking separate eras through artifacts, genetic memories, and ancient conflicts.
Important Isu figures include Minerva, Juno, and Jupiter. Their civilization was devastated by a solar catastrophe, after which humans survived and eventually became dominant. The Isu are not the same as the Templars or the Assassins: those are human organizations that emerge much later in history.