The fictional precursor species in Assassin's Creed, also called the First Civilization, is the Isu.
The Isu are an advanced civilization that existed on Earth before recorded human history in the series' mythology. They created humanity and developed powerful artifacts, including Pieces of Eden. These artifacts influence many historical storylines and help connect the games' ancient past with their modern-day conflict.
The Isu were once known in the series by several labels, including the First Civilization and Those Who Came Before. Individual Isu figures include Minerva, Juno, and Jupiter, who appear through messages, visions, or historical revelations. Their society was devastated by a solar flare, after which human survivors inherited the planet.
The Isu are not the same as the Templars or Assassins. Those two are human factions whose conflict continues through history. “Sage” is another distinct term, referring to human reincarnations or genetic manifestations associated especially with the Isu Aita.