This book is the basis for a new core class variant of the traditional surface druid. Applying the same concepts of protectionism and guardianship to the lands beneath the surface of the world the book crafts an interesting twist on a deadly predator. A lot of the material, particularly the spells, in Guardians of the Underhalls has been offered to the community before in one sourcebook or another. Section 15 of the Open Gaming License printed at the very back of this book will shed some light on some of the potential sources for this material. The book is not entirely a compilation however. The class itself, its new twist on wildshaping, and the poisons are just a few of the original content that was blended with the original OGC to create this book.
This book does not try to give you new rules for the sacred rituals performed by a coven of druids or for rituals made possible by special groves. Several other books on those types of subjects already exist in the marketplace, and are an excellent source of creative ideas if you wish to expand your UnderEarth druidic campaign in that direction.
In some places throughout the book new rules are offered that would supercede those presented in earlier supplements or editions of the game. In each case an attempt was made to clearly mark these changes so that they can be evaluated by your DM for inclusion in his or her game.