These are the things Xing forgot:

Once there was an offshoot of the Disciples of Sanctus Medicus who infiltrated not just the Xianzhou but also several positions within the IPC. These Disciples worked with a goal in mind that was different from the others of their shared faith: to bring about a phyiscal incarnation of Yaoshi who would be able to grant them true immortality. Through way of careful machinations this branch of the Followers of Abundance eventually managed to obtain something special.

Remnants of an Aeon, though what Aeon they didn't know.

It caused a small rift amidst the Disciples, with some splitting away for they had no guarantee the Remnants were connected to the Abundance.

But those who remained were stubborn. "What does it matter if it is possibly not Merciful Medicus' essence?" they declared, "As long as the vessel is perfect, all will be well." And so with that in mind they returned to their hideout, a long abandoned lab in the midst of the Luofu that had once belonged to the Alchemy Commission. For countless years they toiled. And in the end two vessels were created. A male body and a female body. 

What became of the first, the one known as Aeonic Vessel #2, was not to be found in the records the Cloud Knights recovered following their raid on the site. 

Initially the Ten Lords Commission decided to have the remaining Vessel destroyed. But the current leader of the Alchemy Commission refused, arguing against their declaration for months on end until, finally, they relented. The Foxian Alchemist would be given custody of the Vessel, whom he named "Xing", to educate and raise as a proper member of the Alliance. And so he did, raising the immortal boy until his body ceased aging after reaching adulthood. 

It was then the Stellaron Hunters intervened. 

Kafka spoke to Mingze, convincing the Foxian to let Xing go with her and the others. "It is already written, all that you can change is how he'll see you last," she explained. Understanding what she meant, Mingze told Xing to leave with the Hunters, and promptly retired from the Alchemy Commission and disappeared from public eye. But Xing was unaware of that little note.

Xing stayed with Kafka and the others that followed Elio, learning how to fight at the hands of Blade until it was all muscle memory, no matter the style or weapon. 

From there he was taken to Herta's Space Station... 

And the story begins anew from there.