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.
Restored power to the Administrative Block
Drain the Sewers
Activate the sprinklers in the Male Ward
Collect the key in the Female Ward
Collect 15 documents and complete 15 recordings
Finish the game
Collect all documents and complete all recordings
Finish the game in Insane mode
Drop the corpse in the elevator shaft
Finish the game without hiding under a bed or in a locker
Finish the game in Insane Mode without reloading camera batteries
(DLC) Turn off the gas
(DLC) Turn off the electricity
(DLC) Finish the Whistleblower DLC
(DLC) Finish the DLC in Insane mode
(DLC) Collect all documents
(DLC) Complete all recordings