This special pricing is valid through tomorrow, Sept. 16 at 1 p.m. EST. Unlike Steam's recent XCOM promotion, which saw that game becoming free to play for a full weekend, after which it was available to purchase at a discount, this deal gets you a full copy of the game.
Frictional Games is the developer behind the first-person survival horror game, which is good for a weekend of high-quality scares. Their next release is SOMA, which takes The Dark Descent-style gameplay and transplants it into a sci-fi space setting. That game launches Sept. 22.
Amnesia: The Dark Descent also received a sequel, called Amnesia: A Machine for Pigs. We took a look at that game, which wasn't developed by Frictional, upon release.