[Towards the end of the game, you're supposed to be curious enough to google "Knights of the Buried Chambers." From there, you'll eventually find a website (actually an Alternate Reality Game) containing several difficult puzzles and documents to decipher. This will give you clues on actions you can do ingame, which will unlock some secret achievements. Unfortunately, this website doesn't exists anymore, so it's almost impossible to find the solutions on your own. However, achievements can still be unlocked.]
This one is called "The Scepters of Emprise".
- First, you must have unlocked and finished the very last level, "The Well".
- Then, you need to play on a clear day during a solar eclipse (in real life). You'll probably have to change the date on your computer if you don't want to wait for years :). April 8, 2024 should do the trick, but any other solar eclipse date may work too. [NB: solar eclipse will not be visible ingame, so don't worry about it].
- Now, go to "The Study" and find the First Purpose Plaque. As soon as you get closer, achievement should unlock.