In a word, SPOOKY!
Undying oozes with atmosphere. It's like being dropped into the middle of a horror movie, except you're no passive observer here. Through traditional weapons and magic you can take out all the creepy crawlies that the game throws at you. What set this game apart for me was the setting. An old Engligh estate that is haunted up the ying yang by the warped and cursed owners of this sprawling home. I loved discovering all the areas of the mansion. Light and shadow contribute hugely to the overall feel of the game. The graphics are very good and the character models really do come to life, or to put it more accurately, they come back from the dead.
If you're into games that give you the heebie jeebies, put Undying on your list. It maybe old school now, but it's still worth playing.