The ancient precursor civilization in Assassin's Creed is called the Isu.
The Isu are an advanced civilization that existed long before recorded human history. They created many Pieces of Eden, including the Apple of Eden, and used humans as a labor force. Their civilization was devastated by a catastrophic solar event, after which human societies became dominant.
Earlier games often referred to the Isu as the First Civilization or Those Who Came Before. These names describe the same fictional civilization rather than separate peoples. The Isu remain important across the modern-day and historical storylines because their technology, genetic legacy, and surviving sites connect characters from different eras.