One of the classic changes in Dark Sun was where the power for arcane magic comes from. In many normal fantasy settings wizards seem to tap an unknown but unlimited supply of energy to wield their spells. This is not the case in Dark Sun. All arcane magical power is derived from life force. Initially wizards extract it from plants and microbes but as they grow in power they can start extracting it from people. Defilers do this power extraction in a fast and dirty way destroying the life as they do it while Preserves take a more measured approach of drawing from larger areas more slowly to get the power they need.
The use of considerable defiler magic in the past has left the world a wasted land so the populace is not happy with wizards in general. In the original version of Dark Sun Defilers and Preserves were separate classes with a faster advancement rate for defilers to represent their easier path to power. When a defiler cast any spell near by plants would be destroyed out to a distance depending on plant density and spell level. Creatures caught in the range would feel pain and have their actions delayed segments as they fought through the affect but no long lasting affects until you dealt with defilers on the level of the Sorcerer Kings who would actually do hit point damage to creatures as well.
In the new version defiling is a choice a wizard can make we he needs a little extra power for a spell, this is not a bad idea as the fluff had various characters doing both for this type a reason. This is accomplished by a arcane class feature that works with Arcane Daily Attack Powers. When using a daily arcane power, you can choose to defile and get a reroll as a free action if you do not like the first roll. This can be for either the attack or damage roll. This defilement not just kills plant life but does damage equal I think 1/2 their healing surge to all your allies within 20 squares. This is sort of a problem. They claim it is because these people share a link with the defiler but I just do not like it at all. I think if it is going to do damage it should be to all corpereal entities within the distance at a lower amount like the half the level of the daily attack power.
Defilers can then pick up some feats that allow you to extend this affect to other things. While that seems interesting having to spend a feat to take the easy path does not really make sense.
One allows you to defile with at will or daily powers that you activated with an action point.
The single worse thing associated with the defiling power is the psionic defiling feat which allows you to defile to reroll psionic daily attacks. This is just wrong and makes no sense within the world structure. This could be a class feature for the Dragon Epic Destiny as they used to be powerful defilers and psionicists but not a general paragon level feat.
While the mechanics are interesting since I bet missing with a Daily Attack Power stinks, I really do not like the final form.