In the first age, the fiendish children of Khyber rose and dominated the world, until the dragons and the couatl discovered the Draconic Prophecy and gained the tools to fight the fiends. This conflict ultimately culminated in the couatl sacrificing themselves to form the Silver Flame, a gestalt celestial force that imprisons the fiends.
Prior to their sacrifice, however, the couatl fought the fiends across the world. Originating in Sarlona, the couatl set up outposts across Eberron, and the greatest of these outposts was the fortress city of Krezent, located in what is now the Talenta Plains. Krezent extends down into Khyber, and by the time the couatl formed the Silver Flame, they had bound fiendish forces below it. A handful of couatl remained behind to guard it, but they were too few in number to guard every seal, and so they turned to mortals. According to their own records, a small group of humans from Khalesh, in Sarlona, dedicated their lives to the service of the couatl and the guarding of the ruins. Over time, this devotion and proximity to the magic of the couatl transformed these mortals in body and soul. By the time the Empire of Dhakaan rose to power on Khorvaire, the guardians of Krezent could no longer be considered human. They called themselves Shulassakar, the feathered serpents.
Shulassakar appear like humans blended with the couatl, they have serpentine features and their bodies are adorned with feathers that glow with an inner light. They are physically similar to the yuan-ti, but they have little connection to the yuan-ti culturally.
They have a closer connection to the Silver Flame, and the couatl that formed it, than any other species on Eberron, including the Masvirik'uala. That said, much like the Masvirik'uala, this doesn't mean that they are friendly, and the halflings of the Talenta Plains avoid Krezent, and tell tales of the serpentfolk within. The Shulassakar are dedicated to their ancient conflict, and fight darkness with ruthless efficiency, and notably, they will make any sacrifice necessary for the greater good, including the lives of innocents. None can be allowed to threaten the seals of Krezent. The Shulassakar generally will attempt to repel intruders peacefully before they reach deeper into the ruins, but those who refuse are dealt with swiftly and brutally.
The Shulassakar believe that their serpentine appearance is a sign of their bond with the couatl, and that they go through cycles of reincarnation when they die. There are several subspecies of Shulassakar, and they believe that these different forms they can take represent their spiritual growth as they reincarnate.
Bloodsworn Shulassakar
The bloodsworn are the most human-like of the Shulassakar, and can pass for human if they need to. They lack the serpentine features of their cousins, but their skin is covered in beautiful, colourful markings. These markings can be passed off as tattoo's, but they are magical in nature and can change colour and shape depending on the shulassakar's mood. A Shulassakar Bloodsworn could be handled mechanically as an Aasimar, if one were to play one as a PC.
Flametouched Shulassakar
The flametouched are a blend of couatl and human. They have serpentine heads, and their entire bodies are covered in scales and rainbow-coloured feathers. Some of them even have the rainbow feathered wings of the couatl, and can fly. Many of them are clerics and can channel the power of the Silver Flame.
Transcendent Shulassakar
The final stage in a shulassakar's evolution, Transcendent's are the most serpentine of the Shulassakar. They have the head and lower body of a snake, and their entire body is covered in scales and rainbow coloured feathers. All transcendent shulasskar have rainbow feathered wings and can fly, and all of them are powerful divine spellcasters.
Shulassakar Choir
A Choir is formed when a group of transcendent shulassakar willingly sacrifice themselves, performing a ritual based on the original ritual performed by the couatl, when they sacrificed most of their species to form the Silver Flame. This ritual fuses the bodies and souls of the shulassakar to create a gestalt entity, one possessing great power. A Shulassakar Choir is functionally immortal, they don't age and don't experience hunger or thirst, though they can be killed.Forming a choir means sacrificing one's identity, and it's not something that most shulassakar aspire to. Choir's are not created haphazardly, they are created for a purpose, whether it be to a guard a location or to gain the power needed to face a truly terrible foe.