If turning into an Athasian Dragon is the natural end point of an epic-level Defiler-Psion, then the Avangion is the opposite end of the spectrum: a powerful Preserver fully internalizing their psionic studies, and transforming into a being of energy and light.
While statted out back in 2e alongside its Defiler counterpart, it's pointed out that unlike the Dragon of Tyr, the creation of an Avangion is just a theoretical possibility; no one has actually completed the necessary steps to turn into one as, much like the phases of turning into a Dragon, each phase is fraught with the chance to outright fail.
Not for the lack of trying however. So far, there's only a single named NPC who's working towards becoming an Avangion: Oronis, the Sorcerer-King of Kurn, the only of his fellows who felt remorse at the atrocities he committed under Rajaat, and worked to truly make up for what he did. Depending on the source, he's already halfway there, or managed to completely complete his apotheosis into one.
If you're one of the ones working under the idea that Oronis actually succeeded, then his very existence becomes a veritable game-changer for Tyr, and the setting in general, as it represents the first real chance for the environment of Athas to completely recover. Not that it makes things any easier -- not only is the Dragon of Tyr still around, but Oronis would stand alone against all other Sorcerer-Kings and their forces. But in Athas, some hope is better than no hope at all.
Naturally, something this awesome couldn't be left out of Dark Sun's 4e revival, and so the Avangion appeared as an Epic Destiny alongside its Athasian Dragon counterpart (here called the "Dragon King" epic destiny). Membership was simple; reach 21st level in any Arcane Class.
At level 21, it gives you the features Avatar of Preservation (you can't use Arcane Defiling anymore, but you become unaging (so you'll never die of old age) and you gain a bonus on death saving throws equal to half your highest ability score modifier) and Perfection of Mind (increase your choice of two stats - Intelligence, Wisdom or Charisma - by +2 each). At level 24, you gain the Avangion Transformation feature: this gives you a Fly speed equal to your movement speed, low-light vision, the ability to speak Supernal, and the ability to understand all spoken & written languages. Also, as a free action, you can shed bright light for 5 squares; whilst this aura is up, you can add the Radiant damage type to your attacks. If that's not enough holy light for you, at level 26, you gain the Daily Utility power "Wings of Gold", which creates a 5 square aura of holy light around you. In this aura, enemies with Vulnerability: Radiant count as having Vulnerability: All, your allies that start or end their turn in your aura can make a saving throw to end any effect on them, even if a save normally wouldn't remove it. Bloodied & dying allies that start in the aura of your Wings of Gold gain 10 hit points. Its ultimate feature, level 30's Avangion Rising, means that whenever you or an ally that you can see drops to 0 or fewer HP, you can spend a healing surge as a free action to heal that creature. If you do this, the target immediately gains hit points equal to your healing surge value plus your highest ability modifier.