Eternal Darkness is a very interesting experience. The main features of this game are the mental sanity and the story. Talking about the game and how it plays, the mental sanity is the most important thing to care about... It will influence your adventure, you must try to remain as sane as possible, otherwise you'll start to have hallucinations and to suffer in terms of health just by seeing enemies!
The hallucinations are a very cool effect in the game and they fit in it very well. The main preoccupation before the release of the game was in fact this one, that the hallucination could be implemented badly... Well, this has not happened, since they give this adventure a unique characteristic!
The story also is excellent. Very detailed and very well directed through the game, there is attention to every single particular, creating a deep plot that involves the various characters you will play. The game itself is quite easy, there aren't very difficult parts, yet it is an extraordinary experience.