Completed Among the Sleep is an achievement that can be unlocked in Among the Sleep.
How To Unlock[]
This is story related and cannot be missed. All you need to do is play through the game’s five chapters (the ‘Prologue’ is not required for this). This achievement will unlock once you have completed the game.
After you complete the game, there is an additional chapter called ‘Prologue’. It is only accessed from the main menu by choosing Chapter Selection then Prologue.