Epic level rules for Dark Sun in 2e. Actually, it's a little more complicated than that, since it includes the rules for making one of your characters into a Dragon (as obscenely powerful as it sounds, but takes a lot of work). There is something else that's just as relevent, but I'm not going to be the one who spoils it for you.
Basically, if you're running a Dark Sun game, you'll want this sooner or later. If you just want to read about the setting, then it's still greatly reccomended. Great product, as long as you don't hate old D&D.
[4 of 5 Stars!]