Here is the full list of all 25 Coffin of Ashes achievements.
Solve the math puzzle to unlock the door
Go back and get the letter
Save game progress over 30 times
Watch all bonus CGs
Solve the puzzle in the kid's room
Accomplish the ending "Farewell"
Accomplish the ending "Going in Circles"
Accomplish the ending "Rebirth"
Accomplish the ending "Ashes in Coffin"
Open the final door of the six rooms
Lost life in the Blood Pond
Killed by spikes
Die because of the unknown liquid
Collect all coins for the rat
Help the maid find her friend
Distinguish the best spice
Rest in the cabin to refresh yourself
Win the “Ghost Hunt” game with the Lena twins
Meet the other two protagonists
Solve the crystal's puzzle and awaken Delilah
Read all the missing pages of the diary
That was too close to the evil spirit...
Open all the boxes without dying
Put the book where it belongs
Find the hall to understand more about the people