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