From the web site:
" The Diamond Fortress™ is a vast structure composed of polished shards of extraplanar crystal that resembles a massive diamond ripped from the heart of the earth and pulsating with evil power. Inhabited by horrid crystalline demons intent on destroying the Material Plane and transforming the world into lifeless crystal, only the PCs have a chance to stop the fortress before the demons unleash their terrible army and tear the land apart.
Face new monsters and discover terrible new magic items.
Stop the inhabitants of the Diamond Fortress before the land is transformed into lifeless crystal.
Contains abundance of additional D20 material, including never-before-seen monsters and magic items.
Presents players with a bizarre and dangerous gaming environment, which threatens everything they hold dear."