Here is the full list of all 50 Insane Decay of Mind achievements.
Discover the scratches on the wall
Discover the creators of the Labyrinth
Discover the misterious smoking guy
Discover one of the thruths
Discover the torn map
Discover where you are
Discover the broken pipe
Discover what happened to the structure
Discover the chained Lagomorph Shadow
Discover the St. Anger Manor model
Discover you scratching yourself
Discover the old looking car model
Discover why it's cold
Discover Jace the thief
Discover uncle Charles visiting you
Discover the Colossal Shadow trying to kill you
Discover Abe the speaker
Discover the Circus model
End a story Mode
Discover the date of the present
Get killed by the Pale Shadow
Get killed by the Creeping Shadow
Discover your mates' quarrel caused by the debt
Discover yourself eating a rat
Get killed by the Colossal Shadow
Get killed by the Childish Shadow
Discover why you are in this structure
Get killed by the Monster of the Labyrinth
Get killed by the Lagomorph Shadow
Discover the three version of yourself
Start a new Game for the first time
Discover all the secrets hidden in the Labyrinth
Discover the lifeless body of the old landlady
Discover the rebel of the school
Discover yourself stabbing the Colossal Shadow
Discover the strange code
Discover the face of your teacher
Discover the strange note
Discover the bricklayer working
Discover Miss Janet
Discover the soldier mate
Discover your mate's memories
Discover something from the future...
Discover your uncle dying
Discover your alter-ego
Discover Miss Janet looking for a book
Discover the Mason twins
Discover something awful
Discover your home model