'Demon Stone' isn't a bad game, but it isn't extremely innovative, original or even very interesting for the matter. Simply put it serves a dish that has been served many times before it and will be served for many, many times more.
As for plot goes 'Demon Stone' comes from the most unoriginal end of the town, with eons ago banished demon gods, trolls and other monsters, lazed up with the all the typical mush writer R.A. Salvatore, who also wrote this one, usually serves in most of his D&D books.
Gameplay is pretty simple run and hack. No thinking required, just run and hack everything you see, and in the end of the level buy some new gear to become even better killing machine. You can freely change between three different characters, in one level you get fourth one, but that is just for that one level. Of course all the characters have different skills: mage, fighter and a thief. Luckily they all differ from one another, so the hacking isn't that monotonous. Besides each level is pretty short and more experienced, and maybe even a bit more inexperienced, will complete 'Demon Stone' in a day or two. If they bother that is, as there are so much better games out there.
As I said, there isn't actually anything in the game that would make it stand out of the mass. And the length of the game really doesn't warrant a full prize tag either. Luckily I got this one in a collection of three other games from a bargain bin. So if you can get it for cheap and don't have anything better to play, try it, but you don't miss a thing if you don't.