The First Civilization in Assassin's Creed's modern-day lore is called the Isu.
The Isu are an ancient, highly advanced civilization that existed before recorded human history. They created humans and constructed powerful Pieces of Eden, including the Apple of Eden. Their conflict with humanity and their attempts to survive catastrophic events form the mythic foundation beneath the series' historical stories.
Older games often refer to these beings as the First Civilization or Those Who Came Before. Those descriptions are labels for the same broad group, while Isu is the later canonical name most closely associated with them. The term should not be confused with the Templars or Assassins, who are human organizations formed much later.
The Isu storyline links eras that otherwise have different protagonists and locations. Messages, artifacts, visions, and modern research gradually reveal their history. Origins, Odyssey, and Valhalla expand the mythology, but the Isu remain distinct from the human characters whose memories are explored through the Animus.