I'll get this out of the way quickly, because it irks me but it is totally irrelevant, really: To me, Amnesia in a game context will, forever and always, be the brilliant text adventure written by Tom Disch, the equally brilliant author of On Wings of Song, one of my favorite science fiction novels, as well as of The Puppies of Terra, which I have always thought was the inspiration for Porno for Pyro's We'll Make Great Pets.
The original working title of this game was apparently Lux Tenebras, a take on Lovecraft's motto for Arkham University ("Ex lux ad Tenebras," meaning "Out of light into darkness"). While the modern title is excusable since the amnesia of its protagonist is a major device, the older title would also be a propos, since (in common with many horror titles) the lighting is often quite dim -- and indeed your character slides into 'madness' if the lighting is inadequate.
The Dark Descent is created by Frictional Games, who previously developed the Penumbra series. From it, they inherited a graphic adventure engine incorporating real physics -- and an increasing facility at creating physics-based puzzles for adventure games that rely less, if at all, on the "guess the designer's intent" problem that affects older adventure games. That is, you can use aspects of the environment to solve problems -- piling things up to reach heights, or leaving a trail of books you pick up to mark your path -- instead of always having to guess that combining item A with item B will produce item C that neatly solves your problem. There are still some inventory combination puzzles, but they are fairly logical in nature.
The Dark Descent departs from adventure gaming convention not only in the use of physics, but also in the quasi-action nature of much of the game. Often, you encounter monsters and, since you are unarmed, you must either hide or evade them. Hiding means going into shadow, which decreases your sanity (of which more in a bit); flight is often advisable. Moreover, your character is not 100% under your control; quite often he breathes heavily or whimpers in fear, so that you find yourself doing things like facing away in order to reduce his fear and thus your chance of being discovered by an enemy -- and then hearing movement behind you, which increases your unease.
The Dark Descent borrows from Call of Cthulhu the notion of "sanity" as a measurable characteristic that can affect the game; in moments of stress, or after exposure to darkness, your vision will blur or waver, and the game at times takes immediate control of motion away from you, representing your character's terror or madness. This of course reinforces the unsettling nature of the work; prose can unsettle only by description, while in a game for the controls not to function as you expect because your character is unsettled is itself an unsettling and eerie feeling.
The actual story of The Dark Descent is over the top, and by the end, already somewhat absurd in its recomplication; but what's going on here is something more important that this suggests.
What this game is, ultimately, is an amazingly manipulative emotional experience. It is geared, in every way, to unsettle and scare the player -- "scare" is perhaps too strong a word, at least for those of us who read dark fantasy, for no experience of prose, film, or game is likely to actually loosen our bowels; real fear comes from real danger or real psychopaths. But like the best dark fantasy or horror film, The Dark Descent creates a strong frisson of unease, the very emotion we pursue horror to find.
And what's important about that is that the cinematic and literary techniques to induce that kind of emotion are well established; the implication of horror but delay of exposure, dim lighting, sudden revelation, slow build of uncanniness, and so on. There are many conventional videogames that attempt to be horrific in nature but, like Left 4 Dead and its ilk, actually produce no frisson of fear and instead simply provide a "zombie apocalypse" justification for the same old FPS gorefest.
What The Dark Descent does is establish the ludic techniques of horror. Oh, some have been used elsewhere -- the dark palette of Doom III, the breath-holding suspense of hiding in the Thief series and so on -- but this game uses these techniques, adds novel ones, borrows from cinematic and literary tropes, and creates the most horrific and disturbing game of its kind to date.
As an indie game it will not, of course, sell in huge quantities; yet one suspects that in decades to come, many will point to this game as seminal in the development of a particular genre of game -- a genre that, in prose, has variously been called Gothic, weird, horror, and dark fantasy, and may well be called by some other name next year.