The fictional precursor race behind many Assassin's Creed artifacts is called the Isu.
In the series' lore, the Isu are an advanced civilization that existed before recorded human history. They created powerful objects, including the Apples of Eden, and played a major role in humanity's origins and early development. Their civilization was devastated by a catastrophic solar event, leaving ruins, records, and artifacts behind.
The games initially referred to the Isu as the First Civilization or Those Who Came Before. Those expressions remain common in dialogue and fan discussions, but “the Isu” is the later, specific name used for the species or civilization.
The Isu are not the same as the modern Templars or the historical Assassins. Those human factions seek to control or protect Isu technology, making the precursor race central to the series' science-fiction layer.