Perhaps the single most defining aspect of the Dark Sun campaign setting is defiling magic. It is the reason the world is a desolate waste-land. So I think any game set on Athas needs to have a good mechanism to represent this.
In the stories, all magic is cast by the spell-caster first drawing energy from the surrounding vegetation. If the caster is not careful and draws the energy too quickly, the plant life will quickly wither, die, and turn to ash. This can be avoided by carefully pulling the energy, and only taking exactly what is needed. Wizards who did this were called Preservers, those who cared not were called Defilers. Mechanically, any spell caster could preserve or defile, the choice was made at the time of casting. Really powerful magic ("Dragon Magic") could pull energy from creatures, and there was at least one wizard who drew her power directly from the sun.
In the original game, these were two separate classes. My memory is foggy on how it was supposed to work around the game table, but 2E was sufficiently complex that I'm sure this was all covered by the rules. 4E takes a different approach, trying to simplify defiling by giving any Arcane caster with a daily power an At-Will Free Action to re-roll any attack or damage roll for a daily power. Though indeed simpler, it just didn't feel right to me, so I've created a set of house rules to make defiling more representative of how I envision it.
The 4E Dark Sun Campaign Setting book outlines “Arcane Defiling” as a free action which can be used to re-roll an attack or damage roll for a daily power. I’m going to allow this to be used for any roll done in the execution of any Arcane power, but only once per round. (This means anyone who has any Arcane power has access to this ability. ) Further, the re-roll is done at +5 per tier. However, each time it is used all allies in 20 squares take necrotic damage equal to 1/2 their healing surge if the power is a daily power, 1/4 if it is an encounter power, and 1/8 for any other.
Further, use of Arcane Defiling creates an area of Defiled Terrain (as outlined in 4E Dark Sun Creature Catalog p. 134 - usually producing a “Dead Magic” zone, but could produce others depending on circumstances). The area is a burst equal to twice the level of the power for daily spells, equal to the level of the power for encounter spells, and equal to 1/2 the level of the power for at-will spells (minimum size is the square the defiler is standing in).
Lastly, every use of Arcane Defile earns the caster a Defiler Point. I’m still working out exactly what this will mean, but at this time I think this will be significant mostly in social encounters. If you are familiar with Dark Side Points from Star Wars Saga Edition, I’m thinking something similar to that.
I think this gives more of the flavor of defiling. It is not something only done by powerful wizards for powerful spells in the fiction of the world, thus I was unhappy with the original rule. These house rules give a solid benefit beyond just re-rolling, which represents the extra power a defiler can wield. They also extend the ability to all types of spells, which again is more representative of the source material. The biggest difference is that defiling in 4E affects creatures and vegetation, much like Dragon Magic in the original setting. I'm keeping this as part in part to keep the power balanced.
So, what do you think? If you use this at your table, I would love to hear about it.