The Isu are the ancient precursor civilization in Assassin's Creed lore.
The Isu are an advanced species that lived on Earth long before recorded human history. They created many powerful artifacts, including Pieces of Eden, and engineered humans for labor. Their civilization was devastated by a solar catastrophe, after which humans became the dominant species.
The series gradually reveals Isu history through archaeological sites, recovered artifacts, visions, and reconstructed memories. Important Isu figures include Minerva, Juno, and Jupiter, whose messages influence events across the modern and historical storylines. The Isu are also called the First Civilization in much of the franchise's terminology.
A common mix-up is treating “First Civilization” and “Isu” as separate peoples. They are names for the same precursor culture: “Isu” is the species' name, while “First Civilization” is a descriptive title. The Templars and Assassins are later human organizations, not Isu civilization names.