Here is the full list of all 27 Hollow achievements.
Unlock ending A.
Finish game at HARD difficulty.
Finish Hollow game.
Unlock ending B.
Find all collectable radios.
First bullet for Reaper.
Find all documents and notes.
Find all girl posters.
Escape planet dream and get back to Shakhter-One deck.
Get caught by Reaper.
Go trouth all hallucination events in Laboratories.
Encounter Reaper for the first time.
Slain first flying enemy.
Fix generator with fuse.
Fire-off all hands of wanton, than finish it with a headshot.
Slain first Puker.
Collect all available weapons.
Kill wanton using gas tanks only.
Blow first destructible using gas tank.
Slain first wanton.
Kill wanton with single shot in a head.
Sync all generators.
Get killed more than 50 times.
Kill all monsters available in a game.
Finish game at HARD difficulty under 3 hours and do not use Save Discs.
Find way out of Cable Maze.
Kill burning Puker.