Amnesia The Dark Descent
This is an old-school type thriller. Although it's not mainly horror game, it'll have its scares; and will offer beautifully designed levels in an old-fashioned asylum with various puzzles to solve. You will discover what happened to David, and why the mod is called "White Night".
To enjoy White Night, you need patch 1.20 (Justine) installed alongside your Amnesia: The Dark Descent installation.