Couatl

The Couatl are celestials native to Eberron, the last children of Siberys. Much like the children of Khyber, the couatl were born in the first age of the world. They appear as magnificent feathered serpents, often depicted by the Church of the Silver Flame with rainbow coloured feathers, though whether or not this is how they all looked or is an artistic representation of the specific serpent that came to Tira Miron in her vision is uncertain. On top being able to fly, the couatl were talented shapechangers and powerful psionics.
The Couatl originated on the continent of Sarlona, and they are believed to have formed from the blood of Siberys after he was killed by Khyber. The Couatl were content to remain in Sarlona and let the dragons and other mortals explore the world, until the rise of Khyber's children. When the Overlords rose, initially, the couatl fled to the continent of Xen'drik and hid, until they were eventually uncovered by the dragons.
Together, the couatl and the dragons went to war with the demons. Two of their greatest prophets, Ourelonastrix for the dragons and Hezcalipa for the couatl, discovered the Draconic Prophecy and found within it a path to the future, a future without the overlords. The balance of power during this conflict dramatically favoured the fiends, and the couatl realised that the only path open to them was sacrifice, there was no scenario where the fiendish overlords were defeated and they survived.
Working together, the couatl abandoned their individual selves and fused their immortal essence together, creating a well of pure divine energy, the Silver Flame. After their sacrifice, the Silver Flame could bind the overlords as they were individually defeated by the dragons and other mortal species. Since then, various groups that worship the Flame in their own ways believe that it has been strengthened and reinforced by mortal souls.
Whilst they were not completely removed from the world, an individual couatl came to Tira Miron, the couatl are thought to be extinct. In their final days, the couatl made preparations to ensure that the Overlords would remain imprisoned once bound. They gathered together human volunteers from Sarlona, and tasked these humans with guarding their ruins, eventually creating the Shulassakar. The couatl also touched the sleeping minds of the Masvirik'uala Lizardfolk, creating their shared dream. Even today, tens of thousands of years after the couatl's sacrifice, the lizardfolk are guided by these dreams and work to prevent the release of Masvirik, the Cold Sun.