I may have a whole tentative rhulisti worldbuilding/metaplot in my notes for Beyond the Dead Lands and other fanwork 
I interpret the Blue Age Lifeshapers as not necessarily straight up eugenicist totalitarians, more an authoritarian “pacifist” society with vaguely ecofascist tendencies, think a hippie cult gone bad on a societal level. As advanced as rhulisti “technology” was I wanted to keep an archaic/bronze age feel. The Nature Masters ruled through immense levels of gaslighting, social shaming, and doublespeak, disguising a horribly repressive and stagnant society under auspices of “natural order,” and while on the surface the society seemed pacifist and utopian, “do not kill” meant punishment took all different forms of “humane” alternatives, think like, speaking against a nature master has you “taught respect” by having some parasitic organism that prevents your from speaking for a certain time stuffed in your mouth, or your mouth closed shut by flesh, horrors like that. They certainly did kill people (via assassins with tons of hand-washing about how they never killed anyone, it was just the “cycle of life”) behind the scenes, but it was rare and “humane punishments” usually instilled enough fear to maintain social order. In terms of social norms, Rhulisti society was probably a wierd mix of archaic backwards and oddly modern- for example, if you were non gender conforming, you were told that altering your body was a perversion of the sacred rhulisti form, but grafts were fine, as those are “outside” the sacred body. Overall the Nature Masters were supremely racist, not even recognizing their own sapient “allies” such as the Whales and Dolphins as “people” but rather “good servants,” and Rhulisti “ecology” was tainted by the idea that their role in the biosphere was the “brain” with everything else existing to service them: the pinnacle of evolution and sacredness was the Rhulisti form.
Likewise their reliance on lifeshaping led to complete atrophy in many conventional aspects of technology, I envision the Rhulisti, before they developed lifeshaping, being more a neolithic/early copper age maritime society that relied on the ocean, and as a result they hadn’t developed complex metallurgy or sedentary agriculture- imagine if a neolithic society shot to 21st century levels of technology in a few generations, with no experience tempered by any hardships, really, and the absolute mess that would ensue. What were fishing guilds and maritime merchants clans skyrocketed to “biopunk” megacorporations in bed with the Lifeshapers, while the Nature Masters were equivalent to a deeply nepotistic and corrupt academic institution.
Initially the first Elemental Clerics, Druids, Psions, and what became the Life Benders emerged from that academic tradition, as new avenues of exploring the “essence of life” were pursued, and for a time there was a renaissance of magic-lifeshaping cooperation (my attempt to explain the more obviously magic elements of things like the PT and the lifeshaped artifacts in Psionic Artifacts of Athas) Eventually the people in charge perceived these new “sciences” as a threat to their power and repressed or drove them out, psions were hunted down and subjected to “therapy” (lifeshaped surgery to “fix” their brains), clerics and druids were denounced and their assets like the Proto-Pristine Tower seized, though they were prominent enough in Rhulisti society it eventually led to a very tenuous acceptance by the Nature Masters, which is why the Elemental Cults and Lifeshapers roughly get along even among the Rhul-Thaun. The Nature Benders started as the ancient version of Chahn, a sort of student protest movement among younger, more progressive Lifeshapers who wanted to reform their stagnant society and explore new avenues of research.
They didn’t start evil, decades of repression and moral shortcuts led them to become more and more violent and radical, and their individualist ideals slowly corrupted into a sort of genetic objectivism of viewing people as raw material for research and “art.” Their ideals and a shared enemy led a lot of Psions, Clerics, and Druids who had avoided the Master’s crackdowns to join them, and Rhulisti politics led a lot of nations that were not Tyr’Agi, as well as the more cuttthroat mercantile clans, to back them (or sometimes play both sides). The First of Wars was more an evil vs evil conflict, with both sides thinking they were in the right and turning to more desperate and depraved methods to win, and the Nature Masters, as they lost to the Benders, threw any of their supposed morals out the window and turned to torturing magic users and Benders for their secrets and turning to bioweapons much like their enemies were using. In the end the Masters won, and I like the question of whether the Brown Tide was a bioweapon (As Wisdom of Sorrow implies) or some sort of agricultural experiment gone awry (the more canon revised edition explanation), left deliberately vaguely - maybe it was both? Who knows. The Brown Tide is dead and gone and most certainly not a lingering threat down south… In the end, the Nature Masters couldn’t stop the tide and turned to the elemental magic of their former enemies to save themselves, bringing Elemental Clerics in to use the PT to its full potential and burn the seas.
Some of the rebirth beings would be Nature Benders and their collaborators turned as “punishment” (as a nod to Wisdom of Sorrow, like the earliest Zik-Chil, maybe) while others were rebellious lower-ranking Masters who sought to make amends for their crimes. I have the notion the grudges of various groups carried on into the Rebirth- for example, the Tanysh humans are the descendants of a Bender-aligned branch of Clan Taene, who were the Tyrell Corp of the Blue Age, whose members were punished for their war profiteering and treachery by being turned into “freakish” humans. Their sense of broken entitlement and “betrayal” filtered into a human supremacist mindset. Likewise, the ancestors of the Remaan were a mixed group of rebels who seized the Pristine Tower and voluntary turned into rebirth races, and were joined by repentant Nature Masters- this group of interrelated families were tasked with watching over the first Pyreen and developed their trademark multicultural system.
Rhulisti society didn’t collapse overnight, many of the surviving enclaves were undersea cities beached by the receding oceans or left high on the islands that were now mountain ranges- some died off from political strife or the lifeshaped technologies they depended on failing, as they didn’t know how to hunt or farm. Others were destroyed by vengeful or desperate rebirth races and creatures mutated by the PT, and others experienced more peaceful transitions as they adopted new life-patterns and became “feral” halflings. A few cities like the Rhul-Thaun’s villages or Basrai up in the White Mountains survived for a very, very long time.
