The secret precursor race in Assassin's Creed lore is called the Isu. They are also known in earlier series terminology as the First Civilization or Those Who Came Before.
The Isu created the Pieces of Eden, powerful artifacts that include the Apple of Eden and the Staff of Hermes. Their civilization existed on Earth before recorded human history, and they engineered humans for labor. A conflict between humans and Isu, followed by a catastrophic solar event, shaped the distant backstory revealed across the games.
The Isu are not the same as the Templars or Abstergo. The Templars are a human organization pursuing order and control, while Abstergo is a modern corporation associated with them. The word Isu became the preferred terminology in later lore, although the older phrase First Civilization remains common in discussions of the franchise's mythology. Individual Isu figures include Juno, Minerva, and Aita.