Royal Defiler mentions you need to be a defiler. From what I can gather it deals with being an Arcane Spellcaster of sorts, but I am not sure if it is a specific class, a status Arcane casters can obtain with some feat or other prestige class, or just a general label for Arcane casters in general?
In Dark Sun, arcane magic draws power from the life energy of Athas. Good people generally try to avoid taking too much, and are called "preservers". Evil people draw all they can for extra power, which has a bunch of cool mechanical effects (free metamagic, re-using slots) but kills all the nonsentient life in the vicinity - for decades to centuries. Athas is largely a wasteland in the current era because there are actually quite a few evil people around, and defiling adds up fast.
Dragon 315 has rules for preserving/defiling magic, and 319 has a Dark Sun player's handbook (Dungeon 110 has a complementary Dark Sun DM's guide). Not sure whether that's reprinted/superseded by other 2nd-party Dark Sun material.