Haven't tried it myself, but you might want to check out the
review that's on GameFAQ's for it. There's only one there at the moment, but I'm sure more people will contribute soon.
All I really know is that it's an RPG type game where your main character is a vampire. The cartridge has a light sensor in it, and stuff in the game will be affected by the amount of light that you're playing in. I've heard there's also a clock in the cartridge, so it can do stuff based on what time of day it is. You probably already knew that stuff though.
--Zero