Han Sen wasn't surprised by the appearance of the door behind the cogwheels. Instead, he let himself be pulled through.
This was a necessary part of the process of becoming a King. Whenever a King class creature successfully kickstarted their self-universe cogwheel, they were given permission to enter the geno universe core area.
No one had ever been able to determine the exact nature of the core area. Some said that it was actually a special xenogeneic gene. Others claimed that it was a location within the geno hall. There were many other guesses, but no matter which theory was more popular at the moment, there was never any evidence to support one claim over another.
For now, there was only one thing that everyone agreed on: the core area could be accessed through the self-universe cogwheel once someone ascended to King class.