Assassin's Creed Odyssey

To unlock this achievement you need to win your first conquest battle on land anywhere other than Megaris. The likely first chance you get to do this is in Phokis as it's where you're taken to at the start of Episode 3. To trigger a conquest battle you need to destabilize a region by destroying the controlling factions resources, looting their nation chests and killing their leaders. Once you have the leader of a region vulnerable, the conquest battle will become available. You will then find two opposing armies on your map in that region and can head over there to take part. You can choose to fight for Athens of Sparta depending on your chosen loyalties, though the reward and the difficulty isn't always the same for either side. Regardless of who you side with, win the battle and the achievement will unlock.