Cyclop’s Lair Location
The location you’re looking for is known as the Cyclop’s Lair. It’s toward the middle western part of the island and you’ll be expecting to see a lot of enemies roaming around.
You can go there through different sides; some may be easier than others. Personally, I come up from the bottom where there’s less enemies and more paths.
Where is the Obsidian Eye
There are numerous enemies around this location, so make sure you try and tag all of them with Ikaros to give you a better understanding of where you could be to better strike at them.
Don’t forget the two lion cages on the outer portion as well as a single lion cage within the area of the buildings. They can help you distract the enemies while you either take them down, or take the item in question without being spotted.
Note that there is an alternate path to sneak into, but I hardly ever use it. (It looks like a staircase icon.)
The Obsidian Eye is in a small house on the table. You can also find a Treasure in another house.
Fast travel to Lightning Zeus and head to Markos to turn in the quest.