In the Assassin's Creed franchise, the Isu are the First Civilization that created the Pieces of Eden.
The Isu are an advanced precursor species who existed on Earth before modern humans. In the series' mythology, they created humans as a labor force and developed powerful artifacts, including the Apples of Eden and other Pieces of Eden. Their civilization declined after a catastrophic solar event and conflict with humanity.
The Templars and Assassins are later human organizations that seek, protect, or oppose these artifacts. Abstergo is a modern corporation connected to the Templars, so none of those terms identifies the First Civilization. The Isu were previously called the Those Who Came Before in much of the series' early terminology.
Isu characters such as Minerva, Juno, and Jupiter appear through visions, recordings, or reconstructed memories. Their ancient technology explains the franchise's recurring blend of historical action and science fiction, while the games deliberately leave parts of their history mysterious.