From the Back of the Book:
Defilers and Preservers: The Wizards of Athas details the two paths of magic in the DARK SUN campaign and the strange way magic works there. It demonstrates how Athasian wizards power their magic and describes both types of wizards--defilers and preservers.
Defilers leech energy from the land, stealing life away with every spell they cast and leaving only dead ash behind. Preservers also gather energy from the land, but they take only what they need and work to keep the world healthy and alive. These two mortal enemies stand locked in a never-ending battle--with the survival of Athas at stake.
This accessory presents everything needed to create and play preserver and defiler wizards.