The Isu are the ancient civilization known as the First Civilization in Assassin's Creed lore.
The Isu are an advanced species that existed before recorded human history and created many artifacts capable of influencing human bodies and minds. Their technology includes Pieces of Eden, such as the Apple of Eden, which drives important parts of the series' mythology.
According to the games, humans were created or engineered by the Isu and were once controlled by them. A human uprising and a catastrophic solar event changed the relationship between the two species. Only a small number of Isu survived, and later cultures often remembered them through myths and divine figures.
The term “Precursors” appears in some Assassin's Creed material and translations, which creates a common naming confusion. In the modern series' established terminology, however, “Isu” is the standard name for this civilization.