Isu is the name Assassin’s Creed uses for the ancient civilization that predates humanity.
The Isu, also called the First Civilization in earlier terminology, are an advanced species that existed before modern humans. They created powerful artifacts, including the Pieces of Eden, and played a major role in the deep history behind the Assassin–Templar conflict. Their society declined after a catastrophic solar event and a rebellion involving humans.
The games gradually reveal Isu history through artifacts, visions, ruins, and modern investigations. Characters such as Minerva, Juno, and Jupiter are associated with the Isu storyline, while later games explore Isu figures through mythic or symbolic presentations.
“First Civilization” is a genuine alternate term used in the franchise, so it can appear in older summaries or dialogue. However, the later and more specific name for the civilization is Isu. “Ancients” is instead associated with the Order of the Ancients, a human Templar precursor organization in Assassin’s Creed Origins.