Defy Authority Achievement in Assassin's Creed Origins
Defeat a Phylakes.
How to unlock Defy Authority
Phylakes are very strong bounty hunters who are much harder than regular enemies. They are spread all around the map and are a fixed level. They will hopelessly out-level you initially, so don’t worry about them right off the bat.
For this achievement, you only have to kill one (though there is a level 40 side quest that tasks you with assassinating all of them which will unlock very high quality weapons and a legendary outfit at the end). Open up your map and look for a red bull icon (you will have to zoom in a bit for them to appear).
Upon killing any one of them, the achievement will unlock.