This achievement can be gained as you progress through the campaign, after confronting The Cyclops
and putting his Obsidian Eye
into a goat's butt hole as part of a cutscene. After doing so, you need to kill goats on the island of Kephallonia
and loot their bodies until you obtain the Obsidian Eye item back.
If you missed this earlier in the game, you can always come back to the island at any point, and there is a goat spawn high on the high by where the mission initially took place.