Arcane powers are hazardous on Athas.
Long ago, no one knows for certain except maybe the Sorcerer-Kings and Rajaat, Athas bore a defect. This defect allowed for powerful magic, enough to destroy kingdoms and raise armies out of nothing. However, those who used such power were doomed to blight the land.
While defiling is not culturally acceptable most defilers have to seek out power on their own, except Sorcerer Kings though they would gladly see a defiler die than master the art.
Magic used by defilers is gathered from plant life. Using too much kills the plant and defiles the soil so nothing will grow.
Those who master defiling are able to shed their original form and become dragons. Currently there has only ever been one dragon Borys, though almost every Sorcerer-King hopes to reach full dragon-hood.