The Irsidu were masters of life magic. They were adept at curing diseases, and consequently enjoyed better health than most subsequent civilizations. They were also experts at teratogenesis, and are responsible for creating most of the magical creatures of the Locus. They are also credited with the invention of telehorticulture.
The most lasting legacy of the Irsidu civilization were the morphed races. These were humans who transformed into partially animal forms in order to gain extraordinary physical abilities. So long as the Irsidu civilization persisted, these people could change back to human form whenever their animal forms became inconvenient, or change into different animal forms. However, once the Irsidu civilization collapsed, many of its secrets of teratogenesis were lost, and those who had assumed animal forms at the time of collapsed became trapped in their half-animal form.
Of the morphed races, those who were in half-serpent or naga form retained some shapeshifting abilities. This is likely because their serpent nature allowed them to molt, and part of the molting process allowed them to shed their old form along with their skin. These shapeshifting nagas retained enough of the old Irsidu culture that they continue to refer to themselves as Irsidu to the present day.
Early in the history of the naga Irsidu, there was a major political split over how to deal with the surrounding normal humans, who were generally antagonistic to all morphed races, nagas included. To this end, the naga Irsidu developed two major camps. One, the Peace Irsidu, sought to earn the trust of normal humans by abiding by strict vows of pacifism. The theory was that if the Irsidu never killed anyone, people would stop regarding them as monsters. The other, the Death Irsidu, sought to use the skills they inherited from their civilization of origin to battle the normal humans any way possible.
In the long run, the Death Irsidu eventually died out or moved into hiding, while the Peace Irsidu prospered and multiplied. While the Death Irsidu were feared opponents because of their ability to create disease, this ability ultimately gave the normal humans even more reason to hunt them down and destroy them. On the other hand, the Peace Irsidu gained a reputation as skilled healers, and many nations offered them protection in turn for their valuable services.
A second schism occurred sometime after the fall of the Asryn civilization, but prior to the rise of the Srothi. In this era, the Peace Irsidu saw ever-increasing violence and brutality. The nations that had given them refuge were no longer able to do so. Not only did these Irsidu have to fear irrational fear or hatred, but also the fact that they were now targets in warfare: destroying another nation's Irsidu population was a way to damage its health and economy and thus reduce its ability to pose a future threat. Under such circumstances, the Irsidu began to re-think their vows of pacifism. Some form of self-defense was required, but it could not be done at the expense of the trust the Peace Irsidu had cultivated over so many generations.
The result of this was the formation of the Temple Irsidu. The Temple Irsidu were bound by strict standards of behavior, including an elaborate code of honor, but no code of peace. The Temple Irsidu were to remain a minority, but would be responsible for protecting the majority Peace Irsidu. In turn for this protection, the Peace Irsidu agreed to provide the Temple Irsidu with information and material support. This arrangement proved successful, as the Peace Irsidu could be easily recognized as useful and harmless, whereas the Temple Irsidu operated mainly in secret and managed to avoid getting involved in normal human politics.