In what year does Assassin's Creed Odyssey primarily begin?

Assassin's Creed Odyssey primarily begins in 431 BCE.

That date places the game's main story at the start of the Peloponnesian War between Athens and Sparta. Players control either Kassandra or Alexios, descendants of Leonidas I, and travel through the Greek world while becoming involved with the war, the Cult of Kosmos, and the fate of their family.

The game opens with a prologue set at the Battle of Thermopylae in 480 BCE, where Leonidas leads the Spartan defense. The main playable storyline then jumps forward to 431 BCE. This distinction is important because the famous Thermopylae sequence is not the beginning date of the principal adventure.

The abbreviation BCE means “Before Common Era” and is equivalent in numbering to BC. The game also includes later and mythological material, but 431 BCE is the key starting year for its central historical campaign.