One of the many ways that Dark Sun differs from your traditional Dungeons & Dragons setting is that there are no native Dragons. Oh, there are Athasian Drakes, but those are basically giant feral reptiles with innate elemental psychic powers - the Fire Drake, for example, is basically a T-Rex with pyrokinesis. In fact, there is supposedly only one Dragon in Athas - the Dragon of Tyr.
But that's not quite accurate. See, Athasian Dragons are epic-leveled multiclassed psionicist-defilers who have turned their combined magical powers towards transforming into a superior state. The Dragon of Tyr is the just first psion/defiler to complete the transition, and his former compatriots, the Sorcerer-Kings, are all at some level along the transformation themselves. Indeed, becoming an Athasian Dragon is typically something an epic level character in Dark Sun could seek to do, from part of a sourcebook in 2e to a Dragon Magazine epic-level prestige class in 3e to an Epic Destiny in 4e.
Athasian Dragons, needless to say, are nasty, nasty creatures, with immense physical, arcane and psionic power at their command. In their appearances prior to 4th edition, this was somewhat counterbalanced by the extreme requirements needed to progress through the levels to become a full-fledged dragon, and by the fact that most of those levels allowed the Dungeon Master to take control of your character whenever they felt like it. On the other hand, achieving even the lowest level of Athasian Dragonhood meant that your character became ageless, as well as gaining increasingly strong combat abilities, the ability to speak any language, along with eventually gaining a devastating breath weapon of superheated sand.
The ritual that creates the Athasian Dragon inspired the only Sorcerer-King to almost complete it, the Lich Dregoth, to believe it was humanity's ultimate destiny to transform into dragons as a species. His experiments in bringing this about led to the creation of the Dray race.
AD&D Athasian Dragons
The process of becoming an Athasian Dragon is similar but far more destructive than the process to become an Avangion. The basics are identical; acquire enough experience points to reach the necessary level, cast a spell named Dragon Metamorphosis, offer up the necessary resources, and you're another step along the path. Only human or Half-Elven Defiler/Psionicists of 20th level are eligible. One material component common to all steps is a number of obsidian orbs, all of which must be activated and swallowed before casting the psionic enchantment necessary to advance.
Metamorphosis Stages and Requirements
The stages are divided into low, middle, high, and final. As expected of the ascent to the pinnacle of Defiler (any Athasian spellcaster that destructively draws power from plants, almost permanently rendering the ground sterile) power, the details are fairly unpleasant and brutal, even for the one undergoing the transformation who is wracked by incredible pain each time. Of note is that from 25th to 29th level, the Defiler in question goes mad with a lust for destruction due to this pain and the need to complete the transformation, normally resulting in the character spending a lot of time rampaging through the countryside attacking anything alive it can reach (in essence being controlled by the DM).
Low (21st - 23rd level): The Defiler is merely beginning the metamorphosis. The preparation for casting at these levels requires access to ancient documents, tablets, and scrolls that have never been studied by another defiler. Each of the lower-level steps requires that such materials must be studied for at least eight hours every day for an entire year. The material components must include vast riches (at least 10,000 gp worth of jewels, gems, coins, or artistic treasures), a huge structure where the transformation might take place (which can be reused for all transformations of this group), and no fewer than 1,000 Hit Dice of living creatures for the life-leeching process (this last and similar requirements along the way might be one of the major reasons that most of the Sorcerer-Kings rule over large city-states, so as to have a sufficient stock of "volunteers" no one might miss). The riches vanish and the living creatures are slain one heartbeat after the defiler begins casting. The spell is cast from the deep interior of the structure where the caster will actually transform. No other beings may be present at the instant of casting.
Middle (24th - 26th level): The preparation time extends to two years. During this time, the caster visits a powerful creature on an elemental plane for three days of every 15. The material components include fewer riches (at least 5,000 gp) but more(!) living creatures (no fewer than 2,000 Hit Dice). A new structure must be built, which can be used for all three middle level transformations.
High (27th, 28th, and 29th level): The high levels of Dragon Metamorphosis must take place on either an Elemental or the Astral Plane. No structure or riches are required, but the caster must travel to the plane of choice with no fewer than 200 Hit Dice of living creatures from the Prime Material plane. The living creatures must be no fewer than 10 Hit Dice each and must willingly travel to the plane and participate (i.e., die) in the casting. Casting time is 24 hours, and at least three powerful beings from that plane must cooperate for that time.
Final (30th level): This stage requires no preparation time and but a single material component; the slain body of a good creature defeated in single combat. The victim must be intelligent, have at least 20 Hit Dice, and be capable of casting 9th-level wizard spells or 7th-level priest spells, all of which means that if one of the PCs is high-level enough, s/he could even become a target for a Dragon looking for this final component in its transformation. The spell must be cast over the fallen victim within one hour of the defeat; the casting time is one turn.
Because of course there's a monstergirl version