Here is the full list of all 13 Horror Engine achievements.
Fall victim to April Fools' Day prank
Find the Main Menu
Fall into the Abyss
Burst all of the Invader Balls
Find the Passcode
Get the Flashlight
Find the Hidden Button
Get Caught by the Creepy Soul
Solve the Candle Puzzle
Try to shoot the Creepy Soul
Destroy the Creepy Soul
Finish the Game
Become a Portal Addict