The quickest way to get across the idea of Dark Sun is "D&D for the 90s". It's unsubtle, edgy, enjoys turning things up to 11, and is even more permeated by environmental concerns than D&D generally (see: the ever-present Druid oddity).
It is also really fun, mostly because it did turn everything up to 11, but also because it just about hit the TSR D&D sweet spot of "hey, we've learnt a few things about making a game now", while coming before the worst of the frantic times, when they were in deep financial trouble and just ended up spamming out low-quality untested books. (Though, IMO, most of the DS supplements do fall in that period and are best avoided. For example, Dragon Kings suffers from turning everything up to twelve, while the updated box set includes a lot of crazy weird/dumb lore that takes away from the stark lethality of the setting which is its main point).
Your setting is:
Dark Sun, Athas, desert planet. The blood-red sun rises over vast tracts of sand and rock, scorching the land below until it heats to temperatures reaching nearly a hundred and fifty degrees Fahrenheit (over sixty Celsius); at night, the thin atmosphere and minimal humidity mean heat dissipates incredibly rapidly, often dropping well below freezing. At any time, brutal winds are scouring the wastes, and month-long sandstorms strip flesh from bone. Metal-poor and water-poor, life is hard, primitive, and short.
The major centres of civilisation (such as it is) are the cities of the immortal sorcerer-kings, each gathered around one of few known large oases. While these quasi-divine despots rule with an iron fist through their templar underlings, and the walls are constructed as much to keep the people in as enemies out, they still at least have semi-reliable water and harvests. Between the provision of food scraps and the gladiatorial contests put on by passels of the sorcerer-kings' innumerable slaves, these states embody the uneasy contentment bought by bread and circuses.
Outside the cities are mostly small nomadic tribes - traders, herders, and raiders. Elves are the most notable itinerants, but packs of the hive-minded Thri-keen mantis warriors are also a frequent sight. A smaller oasis may host a temporary village made from settlers who want to make lives for themselves neither dependent on the sorcerer-kings nor the vagaries of wandering life, but these are usually swept away by Athas' brutal environment or jealous pillagers.
Magic on Athas is relatively rare, and quite different from that of other planes. For the clerics and druids, no gods will touch Athas with the divine equivalent of a ten-foot-pole. Clerics worship and draw energy from the elements themselves, while druids maintain the futile hope of saving Athas' dying life-force. The templars draw directly on the magic of their sorcerer-king deities for power. Mages conduct their art only in secrecy, hated for their legendary part in the ruination of Athas, and choose either to continue that ruination for quick power, or to progress agonisingly slowly in order not to upset the balance of life.
Virtually all sapients on Athas, however, are psychically talented. Everyone has at least one useful trick which could range from reading auras to synaesthesia to brief time travel to disintegrating people at a glance (if you get your roll right). Others specialise in this use of disciplined, internal energy - quite apart from the dying life-force of Athas - and can do all those things mentioned above and more.
While the sorcerer-kings plot away, the Great Dragon - the only known member of the species on Athas - roams the wastes, feared even by the great cities. Its magical and mental powers are beyond all mortal measure; where other mages draw life from the earth and plants, the Dragon can tear it from every living being. To send an army against it is to have a third of them turned to ash as their vital force is sucked away, another third destroyed by the spells using that vital force, and the last third slaughtered by raking talons and scouring breath.
This is your world: where obsidian, bone, and wood are the materials of all but the most expensive weapons, and these primitive armaments shiver themselves to pieces if used too roughly. Where dehydration is a fact of life. Where the slowly-spreading sands are swallowing life itself piece-by-piece - and will drown you in particular. This is Athas.