Which Assassin's Creed game was the first main entry set primarily in ancient Greece?

Assassin's Creed Odyssey was the first main entry set primarily in ancient Greece.

Released in 2018, Odyssey takes place during the Peloponnesian War in the fifth century BCE. Players control either Kassandra or Alexios, descendants of King Leonidas I, and travel through regions including Attica, Lakonia, Boeotia, and the Aegean islands. The game combines land exploration with naval travel and role-playing choices.

Assassin's Creed Origins came earlier, in 2017, but is set in Ptolemaic Egypt. It introduced Bayek of Siwa and laid much of the groundwork for Odyssey's larger role-playing structure. Odyssey therefore follows Origins chronologically as a release, while its historical period is much earlier.

Valhalla is primarily set in ninth-century England and Norway, and Mirage focuses on ninth-century Baghdad. Those settings can make the franchise's ancient-world chronology easy to mix up, but Odyssey is the Greek mainline entry.