Here is the full list of all 18 Amnesia: The Dark Descent achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Allowed Alexander to escape in Amnesia: The Dark Descent.
Achieved all other achievements.
Picked up every tinderbox in Amnesia: The Dark Descent.
Rescued Agrippa in Amnesia: The Dark Descent.
Some victims died, some lived in Amnesia: Justine.
Not a single victim survived in Amnesia: Justine.
Not a single victim died in Amnesia: Justine.
Read all notes in all games.
Became aware of the merits of A.S. Inc.'s Span-space Device in Amnesia: Justine.
Collected the pieces of the orb and rebuilt it in Amnesia: The Dark Descent.
Killed Alexander and escaped the castle in Amnesia: The Dark Descent.
Failed to escape the cells and was consumed by the Shadow in Amnesia: The Dark Descent.
Lowered the water level in the cistern to reveal a new way out in Amnesia: The Dark Descent.
Taken a rough ride on an elevator in Amnesia: The Dark Descent.
Removed the Shadow's residue in the entrance hall in Amnesia: The Dark Descent.
Escaped the creature in the water in Amnesia: The Dark Descent.
Left when things were getting interesting in Amnesia: The Dark Descent.