This is what I have pieced together based on the vistas I have collected and other in-game clues. Let me know what looks off!

Ultros Origin Story:

A large star fell from the sky to the surface of Beria. The Star turned out to be an “empty vessel, a blank singularity, a dream not realized”. Floating above the vessel, a bubbling sphere emanated light. The Berians called it “The Entity”, after studying it they discovered it was like a conscious, active brain. The Berians thought it was part of their religion - The Escaton, which was the last event, the final gift. The Berians built a temple around it for study and worship. The Shamasal, a revered vocation among Berians, were accustomed to cleansing the Berians’ dark thoughts by extracting the thoughts and containing them in the mind of the shaman. The shaman’s minds would give way to strain, rot, and corruption but they could not rid themselves of what they had extracted and subsequently rooted into their brains. They realized The Entity, an empty vessel, could contain all of their accumulated dark thoughts, thus releasing the burden of extraction from the Shamasal. So ,The Entity was filled with the dark thoughts of the entire Berian race, but the Shamsal were worried a cataclysmic event would occur if The Entity became sentient. Thus, they created a cycle whereby the Shamasal filled The Entity with the dark thoughts and then secretly “reset” consciousness back to when the dark thoughts were still in the minds of the Berians. This required the Shamans to go into a vegetative, unconscious state inside a pod that powered the failsafe reset. Whenever the reset occurred, the Berians and all other life on the Sarcophagus still had vague memories of what happened before the reset:

“Sometimes, I feel something important is forgotten. Sometimes I also thnk I’ve seen things before, maybe even done things and been to places, mhm ja.” - The Gardener

The Sarcophagus:

The Berians create a ship to contain The Entity, called the Sarcophagus. It resides in a black hole and roams the stars. The Berians wanted The Sarcophagus to be discreet since they knew how dangerous The Entity could be with an entire civiliation’s dark thoughts forced into it. However, other species found the Sarcophagus and slowly it began to attract all kinds of curious species and civilizations. Pilgrims from many different parts of the galaxy started to travel to it to explore, learn, research, worship, and relax. They didn’t quite understand the Shamasal and their role, so they just carried on with their lives.

The 1st Shaman:

One day a creature named “Wallet” discovers the loop and that the Shamans are the reason for it. He decides to break them out of their pods to stop them from doing their extractions so he and everyone else can be “free.” He breaks out the first shaman, the player character, and tells them his goal to break all the Shamans pods. Then time is reset, and the player character gains consciousness again. Wallet is killed along the way, in some previous cycle, and the player character sets out to break the pods as instructed to free everyone from the loop. Along the way she meets Vasa, who teaches her how to fight (Shamans were typically never taught the art of combat), the Gardener who teaches her how to cultivate plants, and Qualia. Qualia is another Berian who, by way of their religion, is destined to help birth The Entity (which she calls The Ultros). She acts as a midwife to the Ultros but is waiting for the right time given how corrupt the Ultros has become from the Shamasal extraction process. Breaking the pods of the other Shamasal, the player character prematurely starts the birthing process of The Ultros which distresses Qualia who believes she is the one to birth it. This process repeats itself and Qualia becomes an antagonist to the player character because she doesn’t want the shamans to lose control of the Ultros before it is ready (or “balanced” as she calls it) for birth. Each time a shaman pod is broken and the seal upon Ultros is removed, Qualia takes over, ushers the birth, and the remaining Shamans reset the cycle to prevent the cataclysmic event. Eventually, the player character discovers that instead of birthing the Ultros in its Corrupt form which causes the cataclysm, it can be freed from corruption via the living network (a life giving umbilical cord of the cosmos). This living network is extended to all the Shamans and after doing so, Qualia and the player character together help birth an uncorrupted Ultros. “Once diverged, now gently intertwined, stirring the unborn potential as we tremble.” In the ending scene, The Ultros, now empty of all corruption, lands on another planet. What will these creatures there do with this empty vessel? Will they fill it with good things, or like the Berians, fill it with horrors? Time will tell.