Here is the full list of all 21 Slender: The Arrival achievements.
Escape any level and fall to your death/Slender grasp
Find all scrapbook items
Find all numbered letters
Beat the game in under 45 minutes
Make it through the forest fire without catching on fire once
Beat the 'Into The Abyss' level in under 5 minutes
Beat the game on normal difficulty
Exceed 50 failed pause attempts
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Beat the Genesis level on "Hardcore" difficulty
Exceed 10 failed pause attempts
Beat the game on "Hardcore" difficulty
Disrupt Charlie while he's crying at least 3 times
Listen to all of the radio tracks in the house
Beat 'The Eight Pages' level in under 5 minutes
Pick up an oversized sign and somehow manage to fit it in your scrapbook
Surpass 10 000 total steps taken
Get ensnared by Slender Man’s tenticles
Die in the 'Into the Abyss' level 3 times in a row
Riding a Slide in a Horror Game
Play the piano until the first notes from "No Friends" is played