Here is the full list of all 27 Stay in the Light achievements.
Get 5 treasure objects
Get 25 treasure objects
Get 50 treasure objects
Get 100 treasure objects
Get 200 treasure objects
Solve Puzzle Room 1
Solve Puzzle Room 2
Solve Puzzle Room 3
Solve Puzzle Room 4
Complete the first level
Complete level 5 of the game
Throw the Torch at Him
Survive being attacked, don't die
Get killed in a violent manner
Light 10 Torches
Light 50 Torches
Use your chalk a lot
Speak to all the skeletons
Refuse the offer he gives you at the end
Accept the offer he gives you at the end
Get the first magic mirror
Get the ultimate magic mirror
Complete a level with no torches
Use the Mirror in a way nobody expected
Find and loot the ultimate hidden treasure room
Make it all the way to the end
Get one of each kind of treasure in the game