Here is the full list of all 30 Downfall achievements.
You achieved Downfall or Golden Ending without using tendency hints
You achieved the full, bad ending
You finished the game without using auto-save
You were really mean to Ivy
You achieved the full, good ending
You read the third and Final Part of the letter
You spotted Slenderman in the forest!
You found a penis in the hotel garden
You read Part Two of the letter
The cat ran away from Young Joe
You made Doctor Z really angry
Susan built 2 weapons
Joe found Agnes' head on the desk in the manageress' office
Joe killed Agnes
You found some concept art in the lucky dip!
You were given a motivational speech
You settled the matter with four Sophie's
You completed all the tasks in Helen Road
Joe killed the Receptionist
Joe examines knickers...a valid plot point!
You had a nice chat with Harrison
Joe plays a tune on the piano
You explored all 4 floors of the hotel
You blew out the candle and caused an unfortunate accident
You persuaded the Queen of Maggots to say her name
Agnes persuaded Ivy to join her
Agnes turned on the radio
Don't Cry, everyone dies...
You read Part One of the letter
You completed the tutorial