Assassin's Creed Odyssey

To unlock this achievement you need to find and kill every single member of the Cult of Kosmos. You gain access to the 'Cultists' tab on the menu after completing Episode 3.

There are 34 total cultists that you need to find and kill. On the 'Cultists' tab you can read some details about certain cultists that you have found clues about their identity to then gain access to some details about them and where to start looking for them. You need to follow the clues given to find each cultist then bring them down. This will take the majority of your playthrough to accomplish, culminating in the final cultist being the enemy during the final mission of your story. You will come across the solutions to these clues by completing the main quest missions as well as certain side quests that you will find dotted around the map as well as searching yourself for any that aren't tied to questlines. These are typically, but not limited to, looting certain chests that have solutions or looting certain characters/mercenaries that may have a solution.

Note the cultists are in clusters of six and you need to kill any one cluster of them to unlock the "One Head Down..." achievement. The side quests that are required to find all of these are so many that it would be ridiculous to write them all down, but anything that is shown as this symbol...

...pick these up whenever possible and complete them since they very often lead either to a solved clue for a cultist or another side mission that has a solved clue for a cultist.

An example of a clue for a cultist that does not have quest involvement woud be 'The Silver Griffin' who is a cultist in the 'The Silver Vein' cluster. His clue is 'Information on this cultist can be found in an Attika silver mine'. So you go to Attika on the map and look for a silver mine, there is one in the Silver Mountains area of Attika, so you go there and inside the silver mine one of the 2 lootable chests that make up the objectives for this area has the solution to the clue. You then loot the chest and then the identity of the cultist can be revealed. They then appear on the map for you to go and kill. I would suggest waiting until the very end of the game to clear these up since like I say many of them will be tied to quests that you do throughout the game and only a few left over will need to be specifically hunted down.