Full list of all 17 Outlast achievements worth 1,500 gamerscore. It takes around 15-20 hours to unlock all of the achievements in the base game on Xbox One.
The base game contains 11 achievements worth 1,000 Gamerscore, and there is 1 DLC pack containing 6 achievements worth 500 Gamerscore.
Collected all documents and completed all recordings
Collected 15 documents and completed 15 recordings
Finished the game in Insane mode
Finished the game without hiding under a bed or in a locker
Finished the game in Insane Mode without reloading camera batteries
Finished the game
Collected the key in the Female Ward
Activated the sprinklers in the Male Ward
Drained the Sewers
Restored power to the Administrative Block
Dropped the corpse in the elevator shaft
(DLC) Completed all recordings
(DLC) Collected all documents
(DLC) Finished the DLC in Insane mode
(DLC) Finished the Whistleblower DLC
(DLC) Turned off the electricity
(DLC) Turned off the gas