In Assassin's Creed: Odyssey, what is the name of the secret group opposing the protagonist?

The secret group opposing the protagonist in Assassin's Creed: Odyssey is the Cult of Kosmos.

The Cult of Kosmos is a powerful conspiracy operating throughout ancient Greece during the game's 5th-century BCE setting. Its members influence political and military affairs while pursuing their own vision of order. The player tracks and eliminates cultists across the Aegean world.

Odyssey lets the player choose either Kassandra or Alexios as the protagonist. Regardless of that choice, the investigation into the Cult of Kosmos forms a major part of the story. Clues, masked identities, and cult branches encourage exploration beyond the central narrative.

The group should not be confused with the Order of the Ancients, which appears in Assassin's Creed Origins and Assassin's Creed Valhalla. The Cult of Kosmos is specific to Odyssey's ancient Greek storyline and is not simply another name for the later Templar Order.