This game was revolutionary in every way (pun intended). The context-sensitive commands system(you right click on an object and depending on the object and the the state of the game, you can act on it), the fact that creatures seemingly live their lives independently of your actions and the fantasy setting coupled with an interesting plot made it shine for me.
I was very addicted to this game; it was really scary, the user interface was great (mouse and keyboard driven, you chose how to play the game). The characters were well designed and creepy. One of my favorite games.
