The Isu are the ancient precursor species that created the Pieces of Eden in Assassin's Creed lore.
The Isu are also called the First Civilization and Those Who Came Before. They existed on Earth long before modern humans and created powerful artifacts, including Apples of Eden, to control or influence human behavior.
According to the series mythology, humans were created or genetically modified by the Isu as a labor force. A conflict involving human rebellion, the Isu, and a catastrophic solar event shaped the distant past explored through many games.
The Templars and Assassins are later human organizations, not ancient precursor species. The Hidden Ones are the Egyptian predecessors of the Assassin Brotherhood, while the Isu belong to the fictional deep history that connects the modern-day and historical storylines.