Here is the full list of all 42 Decay: The Mare achievements. It takes around 3-4 hours to unlock all of the achievements on Windows.
Complete the first episode.
Unlock the first door in episode 2.
Complete the buttons puzzle.
Complete the second code lock puzzle in episode 2.
Unlock the metal box in episode 2.
Find all the hidden text in episode 2.
Talk to Jack.
Find all the nails.
Complete the puzzle with the plates in episode 2.
Complete the second episode.
Take your first shot with the camera.
Put the four nails in Teddy.
Find Jack's body.
Unlock the box with the circles in episode 3.
Collect all of the balls in episode 3.
Complete the puzzle with the coloured balls.
Watch the scariest movie ever!
Complete the puzzle with the symbols in episode 3.
Get the knife in episode 3.
Find Tory in episode 3.
Complete the third episode.
Get the bad ending in episode 3.
Use the pocket watch to travel in time.
Reveal the history of Rafael.
Use the magnifier on the painting in episode 3.
Complete the clock puzzle.
Complete the drawings puzzle.
Visit the hidden room with the empty chair a second time.
Find your way through the maze.
Finish the conversation with the bag in episode 1.
Break the brick wall.
Get the sledgehammer.
Complete the fan puzzle.
Get the table near all the photos to break.
Complete the blinking eyes puzzle.
Meet the red butterfly.
Pick up an item.
Get out of your own room.
Collect all coins in the first episode.
Collect all coins in the second episode.
Collect all coins in the third episode.
Get the good ending in episode 3.