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