According to the Assassin’s Creed series, the First Civilization is called the Isu. The Isu are an ancient, highly advanced species that existed before modern humans and created many powerful artifacts.
The series presents the Isu through mythology, genetic memories, precursor sites, and messages preserved in ancient ruins. They created humans as a labor force and used Pieces of Eden, including the Apples of Eden, to control or influence human behavior. A conflict between Isu rulers and humans eventually contributed to a major catastrophe that devastated their civilization.
The Isu are sometimes called the Precursors or Those Who Came Before. These labels describe the same ancient civilization in different contexts, while “Isu” is the franchise’s most established modern term. Their surviving technology shapes the conflict between the Assassins and Templars throughout the series.
The Templars are a human organization, the Hidden Ones are an early Assassin organization, and Abstergo is a modern corporation. None of those groups is the First Civilization.