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