The Isu are the ancient precursor species in Assassin's Creed lore.
The Isu are an advanced civilization that existed before recorded human history. They created humans and developed powerful artifacts, including the Pieces of Eden. Their civilization was devastated by a solar flare and the resulting catastrophe, after which surviving Isu and humans shaped the later history depicted in the games.
Earlier games often referred to the Isu as the First Civilization or Those Who Came Before. These labels describe the same ancient civilization in the series' evolving terminology, but “Isu” became the standard name in later stories. The Isu are not the Templars or Assassins: those are human organizations that emerge much later. Their technology and surviving artifacts connect the historical adventures to the modern-day Animus narrative.