In the Assassin's Creed franchise, which game introduced the Animus as a central story device?

The original Assassin's Creed introduced the Animus as a central story device in the franchise. The machine lets Desmond Miles relive genetic memories inherited from his Assassin ancestor, Altaïr Ibn-La'Ahad.

This framework allows the game to move between a modern-day narrative and the historical Third Crusade setting. Abstergo Industries uses the Animus to search Desmond's memories, while the modern Assassins seek to understand and oppose Abstergo's plans.

Later games expand the technology and introduce different users, interfaces, and organizations. The Animus is therefore not a feature first added in Assassin's Creed II or Origins; those games continue a narrative mechanism established by the 2007 original.