Arcane powers are hazardous on Athas.
Origin
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.
The greatest example of the blight are the Deadlands, created by the Defiler Qwith , under tutalige of Rajaat. She was able to open a gate into the Inner Planes, which spouted a sea of Obsidian.
Dragons
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 dragon-hood.