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