The Isu are the ancient civilization revealed in Assassin's Creed lore.
The Isu, also called the First Civilization in earlier descriptions, existed on Earth long before recorded human history. They created humans and built powerful Pieces of Eden, including the Apple of Eden, which could influence human minds.
The Isu civilization was devastated by a catastrophic solar event. A small number survived, and their artifacts, genetic legacy, and recorded memories became central to the later conflict between Assassins and Templars.
The term can be confused with the First Civilization, which is an older name used by the series for the same civilization. The Isu are not the Assassins or Templars; those are later human movements whose ideological struggle is shaped by Isu technology and history.