Which fictional precursor civilization is known as the Isu in Assassin's Creed?

The fictional precursor civilization known as the Isu is also called the First Civilization in Assassin's Creed.

The Isu are an advanced species that existed on Earth before humanity became dominant. They created many Pieces of Eden, powerful artifacts that influence minds, alter perception, or produce other extraordinary effects. The Apple of Eden is the best-known example.

According to the series' mythology, the Isu created humans partly as a labor force. A conflict involving human rebellion and a catastrophic solar event devastated their civilization. Their surviving legacy shaped later human religions, political myths, and the continuing Assassin-Templar struggle.

“First Civilization” is the clearest alternate name for the Isu, while “Those Who Came Before” is another phrase used in the games. The Isu are not the Assassins themselves, and they are not simply historical humans; they are a fictional precursor species in the modern-day storyline.