#...i caved

The circumstances of Dan Heng's Molting Rebirth and the near-1000 years it took to happen could be considered one of the most troublesome instances of a Vidyadhara birthing process with many of the details obfuscated to time. As it stands, Dan Heng's rebirthing process took roughly 170 years to achieve and he hatched just a few short years before the beginning of the Third Abundance War, born fully-grown but taking years to cognitively reach adulthood. (It should also be noted that there are eggs in Scalegorge Waterscape that have been gestating for 700+ years.)

The following is a rough timeline of this process:

The process first begins with Dan Feng's imprisonment. During this time, Dan Feng was imprisoned, descaled, and slowly harvested of bone marrow and blood to be used as experimental fodder.

He dreamed of the Dracocatena Nails being staked into his body, and chains of corallium winding around him to hang him in midair in the Shackling Prison. He dreamed the elders coming and going to interrogate him about the truth of the Arcanum and the whereabouts of the dragon heart. He did not speak. (...) He dreamed he was raising his cup to drink with the others again, that he molted off his scales, and that he returned to the egg, and became someone else. - Dan Heng IL, Character Story IV

After he came to the brink of death, Dan Feng's heart was extracted and crystallized to become an egg, and his body dissolved into seawater and foam. The remains are still extant in the Shackling Prison and can be heard through the sloshing of water.

Is it an empty room, or does it house an unpardonable prisoner? There's no way for you to find out as the stone door isolates it from everything, leaving only a cold silence behind. But even a door of stone cannot conceal the sound of water sloshing on the other side… It is as if a god that reigned over water is sealed here. - Eternal Prison, Shackling Prison

Dan Feng's egg was kept in stasis for centuries and confiscated by the Ten-Lords Commission while it was deliberated between the Preceptors, Ten-Lords, and General as to what should be done about the prisoner. During this period, the Disciples of Sanctus Medicus and some Preceptors secretly collaborated together and began to experiment with the samples extracted from Dan Feng which led to many failures and an eventual need to replenish what they had expended that demanded a member of the Azure Dragon's lineage.

The experiment involved the injection of Vidyadhara blood and marrow fluid into various species, including those with scales, feathers, fur, long-life volunteers already exhibiting signs of being mara-struck, some short-life species, and outworlder volunteers. The objective was to observe the reactions of these organisms in both their normal state and mara-struck state. - Sanctus Medicus Devils Criminal Evidence Records

Beforehand, Dan Feng had been made to undergo not a Hatching Rebirth, but a Molting Rebirth that entailed the use of an exuviation charm. Therefore, Dan Feng's reincarnation would be reborn with his powers and appearance perfectly intact, except for his Abundance power.

The Vidyadhara were under great pressure. In order to placate the Ten-Lords and Sky-Faring Commissions, they performed an exuviation charm on Dan Feng against his will. Still, they ensured that the charm contained a flaw, thinking that this would fool the Ten-Lords Commission. Hmph, the senior Vidyadhara were sure of their scheme, but you cannot hide fire with paper. - Jing Yuan, Topclouded Towerthrust

At the time of the experiments and the to-be-born Dan Heng's uncertain fate, centuries after the fact, it was decided to release the egg from stasis and to resume the incubation process.

According to the deal between the Alliance and Caelorum Venti of the Yaoqing, the sinner Dan Feng was to be spared from death and instead suffer the punishment of molting rebirth. After that, according to Vidyadhara customs, Dan Feng should have been regarded as a new being and his sins forgiven. - An Appeal from the Vidyadhara

In the period between the early years of Dan Heng's birth and the start of the Third Abundance War, experiments were conducted on Dan Heng as they received a new source of fresh samples: one was by Yunhua who was coerced into perfecting the Injection of Past Life in Dreams. In doing so, it would be administered to Taoran who would be similarly exuviated in a process like Dan Feng but with him able to perfectly recall his past life.

Thirty years ago, my mentor served as Alchemy Commission's Cauldron Master. She recognized the emerging undercurrents and sought to cleanse the source of the disturbance. - Lingsha, Finest Duel Under Pristine Blue (I) But did your mentor tell you… her "good intentions" led her to perform certain healing arts on Dan Heng, who had just finished his hatching rebirth, so that he would regain the memories of his past life? - Jing Yuan, Finest Duel Under Pristine Blue (II)

On the other, the blood and bone marrow facilitated the creation of Bailu, the Bloomborn Scion whom received Dan Feng's Abundance power. Genetically, she'd be the closest the Vidyadhara would have to creating offspring; she would be selected as the next candidate for Imbibator Lunae and become a prolific healer.

There is no need to send new samples after receiving this reply. Back then, we unintentionally created the miracle of Bloomborn Scion (Bailu), but there is a limit to mere luck. Until I make further progress in my research, such a miracle is unlikely to occur again. What we are trying to achieve is difficult. It could change the fate of countless people and even lead us to disaster. However, if we succeed, we will no longer be trapped in the predicament that has plagued us for thousands of years. - Undestroyed Letters

If you want to avoid it, be warned