The original Assassin's Creed introduced the Animus as a playable present-day setting with Desmond Miles.
Released in 2007, the first game alternates between Desmond's present-day captivity and his ancestral memories of Altaïr Ibn-La'Ahad during the Third Crusade. The Animus is a fictional machine that lets a person experience genetic memories inherited from ancestors. Desmond's sessions provide the framework for the historical gameplay and the modern conspiracy surrounding Abstergo Industries.
Desmond is forced to use the Animus by Abstergo, whose scientists want information hidden in his ancestry. The player then controls Altaïr within the simulated memories. This two-era structure became one of the series' defining features, although later games changed how much time they devoted to the present-day storyline.
The Animus was not simply a time machine. It reconstructs memories through inherited genetic information, and the series repeatedly explores errors, synchronization, and the limits of that reconstruction. The first game therefore establishes both the device and the narrative rule that historical missions are filtered through a modern user's experience.