Which Assassin's Creed game introduced the Spartan warrior Bayek of Siwa?

Assassin's Creed Origins introduced Bayek of Siwa.

Released in 2017, Assassin's Creed Origins follows Bayek, a Medjay from the Egyptian oasis of Siwa. The story is set during the final decades of Ptolemaic Egypt and depicts the events that lead to the formation of the Hidden Ones, the organization that becomes the Assassin Brotherhood.

Bayek’s investigation begins after a personal tragedy and draws him into a struggle involving political powers, secret societies, and figures connected to ancient Egypt’s changing royal order. His wife, Aya, is also a central character.

The game is often described as the beginning of the Assassin Brotherhood rather than the beginning of the entire conflict. The Assassins and Templars have earlier roots in the series’ mythology, but Origins explains how the Hidden Ones were founded.