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