The Isu are the fictional ancient civilization behind many mysteries in Assassin's Creed.
The Isu, also called the First Civilization or Those Who Came Before, existed on Earth long before recorded human history. They created powerful artifacts known as Pieces of Eden, including the Apple of Eden, and engineered humanity for labor. Their civilization was devastated by a catastrophic solar event, after which humans became the dominant species.
Isu figures and technology influence the historical and modern plots across the series. Characters such as Minerva, Juno, and Jupiter appear in the franchise's mythology, while artifacts continue to drive conflicts between Assassins and Templars.
“First Civilization” is an alternate description, not a different answer. The Templars are a human organization, and the Hidden Ones are the ancient Egyptian forerunners of the Assassins. The Isu concept provides the science-fiction layer beneath the games' historical settings.