5e Dark Sun Defiling Rules -
Background: Defiling is (or should be) the default method of spellcasting on Athas. It is one of the major causes as to why the planet is arid, barren, and lacks prolific life. No one tolerates defiling magic, especially the sorcerer-monarchs or their templars.
A spellcaster who does not choose to attempt to cast a spell by means of preserving magic automatically casts by defiling magic; a spellcaster may also simply choose to spellcast with defiling magic.
Although defiling magic has the possibility to cause damage to the spellcaster, it is easier to perform, and thus more alluring. Preserving magic, however, is preferred by (mostly) good-aligned spellcasters because it does not cause additional damage, but is more difficult to achieve.
As a Bonus Action: Any time a spellcaster prepares to cast a spell, they must choose from where they want to draw the source of energy from that powers the spell - preserving or defiling.
Preserving: To attempt casting a spell with preserving magic, the character must make an Intelligence ability check against a DC 15 + spell level. On success, the spellcaster is able to draw enough energy from the multiverse (the Weave) so as to not cause damage to the life around them, and the spell casts and functions as normal.
Defiling: If the saving throw fails, or the spellcaster chooses instead to draw energy from living things in the environment around them, they cast the spell with defiling magic. If defiling occurs, all living creatures within 30 feet of the spellcaster, including the spellcaster, must make a Constitution saving throw against a DC equal to 10 + the spell level. On a failed save, the creature takes 1d6 + the spell level (2-15) necrotic damage, and is poisoned until the beginning of the spellcaster’s next turn. Non-sentient plants automatically fail the defiling saving throw, wither and die. On a successful saving throw, the creature is not poisoned.
An Example: My 3rd level wizard is preparing to cast the 2nd level spell, web. He is a heroic character and so prefers to cast spells using preserving magic. On my ability check roll against a DC 17 (DC 15 + spell level), I roll a 15 and add my INT ability score modifier (+2), for a grand result of 17. Thankfully, my wizards casts the spell with preserving magic and no one takes additional damage. The party escapes the bandits who are chasing them.
In the next round of combat, my wizard is preparing to cast the 1st level spell magic missile at the bandits as a ranged spell attack. Again, he wants to cast his spell with preserving magic and so I roll another INT ability check against a DC 16 (DC 15 + spell level), but get a total result of 12. Now my wizard has to cast the spell using defiling magic which means all creatures within 30 feet of him, including my wizard, must make a successful CON saving throw against a DC 11 (DC 10 + spell level), or else take necrotic damage and be poisoned until the beginning of my next turn.
While some of my allies in the party are glad my wizard was able to cast the web spell and thus escape a pursuing bandit, they aren’t so happy that they took additional necrotic damage AND were poisoned because a magic missile was cast with defiling magic. But, nothing is ever fair on Athas.
The benefit of this mechanic is that players will want to attempt casting spells with preserving magic because it does not have the potential to cause damage, or become poisoned, for themselves or their allies.
‘The Veiled Alliance” (Optional Rule)
“Protect the path of preserving, its practitioners, and those who support preservers—especially the Alliance itself.” - dogma of the Veiled Alliance
As an organization which exists to protect preservers from physical, mental and magical attacks by all enemies: sorcerer-monarch, templars, defilers, and general citizenry, as well as being dedicated to preserving all life on Athas, it is possible (or probable) that a spellcaster who becomes initiated into the ranks of this clandestine organization will learn how to better control the manner in which they are able to draw life energy to power their spells, and therefore be able to attempt casting a spell using preserving magic with an INT ability check against a DC 10 + spell level, rather than the normal DC 15 + spell level.
Members of the Veiled Alliance, however, are ruthlessly hunted down by Templars with more passion and enthusiasm than most any other criminal, and are executed on the spot once discovered. So, it’s a legit concern as to whether or not one would want to associate themselves with the Alliance simply in order to cast spells with preserving magic. But that’s up to the character!
’The Order of Templars’ (Optional Rule)
Just as there exists an organization in which preserving magic is sought to be protected, so to is their an organization in which defiling magic is looked upon more favorably - The Order of Templars.
As a spellcaster who defies all odds and is admitted into the privileged meritocracy of the Order, they will become much better trained in the arcane arts. Able to cast defiling magic with more focus and discipline, spellcasting members of the Order of the Templar are able to withstand the effects of the energy draw while casting defiling magic and are thus immune to the necrotic damage and poisoning effects. As a role-playing aspect to using defiling magic, you could institute an increasing level of physical characteristics (such as yellowing teeth, greying skin, puss-filled lesions or blisters, etc.) that (maybe?) produce disadvantage on Charisma based ability checks or saving throws.