I'm a bit torn on how to do defilers in Dark Sun actually, partially because the lore itself is somewhat ambiguous.
Originally in 2e, defilers were just wizards with a faster XP table and when they cast spells they killed all nearby plant life (radius depending on how fertile the area was - in a jungle, you'd only defile a yard or two, but in the sandy wastes the radius could be dozens of yards). In revised 2e they changed it so they defiled when memorizing spells, which I thought was a stupid idea (because the lore often had farming villages having a preserver in their midst because they wouldn't want to destroy their own land, but raiding tribes would have defilers because they wouldn't be casting at home anyway). Lore-wise, on one hand it was said that defiling was easier which is why they had the faster XP table, but on the other it was described as a more recent invention that allowed the users to supercharge their magic. The novels had one of the main character wobble between defiling and preserving (mostly being a preserver, but occasionally defiling when in dire straits), which of course didn't work in game because classes.
4e, IIRC, made "Defile" a power available to any arcane caster, allowing them to reroll an attack at the cost of hurting nearby allies or something like that. That sort of works better with the way it was described in the Prism Pentad (as a "runtime" option), but meshes poorly with other lore.
My guess is that if and when they make a defiler for 5e, it will be closer to the 4e version. Perhaps as a tradition that has the option to gain advantage on spell attacks/inflict disadvantage on saves when defiling. I do hope never to see the idea of defiling when preparing spells again though, because that made no sense.
