Magic in Athas

Magic in Athas doesn't quite operate the same way that it does in the rest of the DnD universe(s) due to the isolated nature of Athas as a plane. On Athas, the only source of energy that can be used to power the arcane arts is the vital energies within the plane itself - life-energy from the surrounding world must be drawn into the caster to power the spells they cast. Obviously, this is detrimental to the landscape, and if not done responsibly, it will leave the ground barren, desolate and unfit for growing anything for centuries. This destructive process is the sole reason for Athas' current, wasted state, and for that reason, the greater populace is incredibly judgmental toward any perceived arcane caster.

Arcane casting practices can be broken up into two categories: Preserver Magic and Defiler Magic

Preserving

The Path of the Preserver is one of patience and control. Arcane casters who are preservers draw the energies they need to cast their spells gently from the land; they either draw their energies more slowly, or from wider-spread areas of plantlife in order to not fatally drain the landscape. They take just enough energy to cast their spells from just enough sources; this way, nothing dies outright, and given a few days, everything drawn upon will recover back to normal. The Path of the Preserver is walked by the expansive Veiled Alliance, an underground network of preserver mages who adamantly and violently oppose arcane casters who practice defiler magic - which, of course, are most notably the sorcerer-kings and their imbued templars.

Defiling

Unlike the above preservers, defiler mages don't give the destruction of their arcane drawing a second thought. Instead of soaking the energies they require from the land, they rip it from the roots of any living plantlife within reach. The process is the same, they simply pull as hard and as fast as possible in order to cast quickly and to maximize their spells' effectiveness. As defilers draw from the ground and cast their spells, a seeping shade of withering decay seeps outward from the point of their arcane pulling. The ground darkens, plants wilt, and, in the case of some powerful defilers, even the animals above the ground suffer and die. The Path of the Defiler is the path of power, but it is also the path that has led Athas to its own damnation.

The Game Mechanics

Defiling or preserving will always be a choice, and the choice will always favor defilers over preservers. Exactly how these options look, however, will depend on the environment at the time of casting.

In high-flora locales (the Forest Ridge, for example), Preservers cast as normal, whereas Defilers cast with Advantage on attack rolls.

In low-flora locales (most of the rest), Preservers must use a move action to draw in their power, and cast with their attack action. They may still utilize a bonus action, however. Defilers cast as normal.

Drawn power may be held, but only until the next round of combat. Magical energy will disperse if held longer than that, and they will not return to the earth.

There are certain types of stone known to inhibit the flow of magical energies. Casters should be aware that substances like these do exist, and may crop up at inopportune times.