The Isu are the ancient beings who created the Pieces of Eden in Assassin's Creed lore. They are also called the First Civilization, and their technology strongly influences the series' mythology.
The Isu existed on Earth before modern humans became dominant. They created humans as a labor force and developed powerful artifacts, including Apples of Eden, capable of influencing minds. A catastrophic solar event devastated their civilization, but some Isu survived or preserved knowledge that later shaped human history.
The Isu are not the same as the Assassins or Templars. Those human factions emerge much later and compete over how society should be organized and how Isu artifacts should be used. Names such as Minerva, Juno, and Aita refer to individual Isu figures, whereas “the Isu” names the broader civilization.