When you use defiling while spellcasting, treat the roll as if you rolled a 12
Defiling drains the nature all around you according to the level of the spell. It also does damage to everyone nearby with every level increasing the range of this. (not 100% sure about the numbers yet)
When you cast a spell while preserving (is that what it was called in Dark Sun?) roll Cast a Spell normally.
On a 7-9 you may also choose to defile.
That is how it works right? Maybe add some countdown thing for the addiction/decay thing of defiling (is there such a thing?).