Elex

In order to unlock this achievement you have to follow the clues that guide you to Calaan, a god worshipped by the Clerics faction in Ignadon.

These clues come in the form of old photos, accompanied by some notes.
You are supposed to find ten of these clues throughout the world-game, then go to a specific location and unlock a door using a code. The achievement will unlock once you get past the door and find out a little surprise.
But if you want, you can just go to this last location and open the door with the following code, without finding all ten clues. That's up to you.

The code to open a door is the following:

The final door is located in an old world building, situated near the Volcano. You can reach this building following two different routes, each ending up with an elevator.
More specifically, you can access the elevator from the Castle ruins of West Ignadon or the Ruins of the Dome in North Tavar.

This video, created by The Hidden Object Guru (to whom goes all the credit), shows the location of all photos and, in the end (3:24), how to access the final building from the Ruins of the Dome:

If there are any mistakes or you have suggestions on how to make this guide better please leave a comment. This is only my second guide published and I'm Italian so there might be some misspelling and other errors.