Here is the full list of all 15 Fear Of The Darkness achievements.
Finish a Nathan's session
Finish a Sarah's session
Finish a Tim's session
Finish a Lucas' session
Finish all of Nathan's sessions
Finish all of Sarah's sessions
Finish all of Tim's sessions
Finish all of Lucas' sessions
Find the first nightmare
Find all the polaroids
Neutral ending
Bad ending
Good ending
Finish the game
You have obtained all the achievements