This is an okay adventure. The premise is pretty interesting, and some of the events are cool. Overall though, I don't think I care too much for it. The main "bad guy" that gets the players involved in this definitely seems to take a "needlessly complicated" approach to the situation. Later when the world starts falling apart, a celestial creature comes down who can help get the players to the stone so they can try to save the world. However, it decides to put them through a series of tests first to see if they are really "good" and heroic. Instead of sending people to save the world ASAP, it decides to waste a lot of time and a lot of its power to make sure your players I guess "deserve" the honor of trying to save the world. That seems pretty rediculous to me. One of the tests also involves trying to make the players sacrifice some magical equipment to keep two Remorhaz magically asleep to show their "selflessness", even though it will probably seem to the players that doing so will only temporarily keep them asleep and that simply keeping their equipment and killing the Remorhaz would be far better for everyone involved, but that wouldn't be "selfless" so it is the wrong course of action. Those kinds of things irritate me to no end.
The way the Stone works is very odd too. Apparently, this world was built so that if this one particular stone is removed from this one particular place, it gets cut off from all the other planes (including the Prime I think), and begins to fall apart. Apparently this is the way the gods made this world. Why? Beats me. Seems like a pretty poorly thought out way to create a world to me.
There are some nice bits. Moloch, a deposed Duke of Hell (from 1E D&D) shows up, and there is a Death Knight who offers to join your group in an effort to redeem himself. There are also rules at the end for playing Death Knight characters. I also liked the idea of how the castle worked in general and some of the events going on in the beginning. Overall though, I think that the basic plot of the adventure and its developement doesn't make a lot of sense or seem very reasonable, so I can't say that I consider it to be a very good adventure.
[2 of 5 Stars!]